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Phloem degeneration in celery infected with yellow leafroll virus of peach
VIROLOGY
6,
Phloem
348-356 (1958)
Degeneration Leafroll
in Celery Virus of
KATHERINE Department
of Botany,
Infected Peach
with
Yellow
ESAU
University of California, Accepted May 19, 1958
Davis,
California
Apium graveolens L. (celery) experimentally infected with the yellow leafroll virus of peach develops phloem degeneration of the type characterized by necrosis of sieve elements, their companion cells, and some phloem parenchyma cells. In the incipient stages of necrosis the protoplasts of affected cells assume a homogeneous appearance and stain somewhat darker than normal cells. Later, the chromaticity increases and reaches a maximum at death of the cells. No hyperplasia was observed in the early stages of disease development, but some mild stimulation of meristematic activity occurred in the procambium in the later stages. After many cells became necrosed a pronounced wound-healing reaction, characterized by enlargement and division of cells, developed about the areas of necrosis. The initiation of phloem degeneration did not occur until one or more sieve elements were differentiated in a given region. INTRODUCTION
Recently Jensen (1956) succeeded in transmitting a virus of stone fruits to celery (Apium graveolens L.) by means of a leafhopper. The celery became diseased in a relatively short time and proved to be a better source of the virus than its tree hosts. The virus used by Jensen (1956) has been recognized in the form of four strains affecting stone fruits. One of these strains, the Green Valley buckskin virus, was found by Schneider (1945) to induce sieve-tube necrosis in peach trees. Some peach tree material infected with the yellow leafroll strain of the virus, examined by the present writer for a preliminary orientation, showed necrosis of sieve tubes and associated parenchyma cells. There was also some excessive development of secondary phloem in leaf veins, a phenomenon resembling that described by Schneider (1945) for the swollen veins of leaves of peach affected by the buckskin disease. Jensen (1956) has reviewed the difficulties encountered in studies of 348
PHLOEM
DEGENERATION
IN
INFECTED
CELERY
349
host ranges of fruit tree viruses and has pointed out the value of transmission of such viruses to herbaceous plants. If such a transmission is effected, it is desirable to know whether the internal symptoms induced by a tree virus in an herbaceous plant are similar to those found in its woody hosts. The present paper deals with the anatomic symptoms induced by the yellow leafroll virus of the peach in the celery plant. MATERIALS
AND METHODS
The inoculation of the celery plants was carried out by Dr. D. D. Jensen, University of California, Berkeley, according to the method previously described by him (*Jensen, 1956). Nine infected plants showing the symptoms of the disease in various degrees of severity and three healthy control plants were available. Shoot apices, petioles, blades, and root apices were killed in chrome-acetic-formalin solution (Craf III) (Sass, 1951, p. 18), dehydrated by means of tertiary butyl alcohol (Sass, 1951, p. 27), embedded in paraffin, sectioned at 10 F thickness, and stained with hematoxylin and safranine (Esau, 1944). RESULTS
The primary internal symptom of the disease was recognized in the phloem tissue. The normal phloem is, therefore, briefly described in the following paragraphs. A detailed study of this tissue in celery has been published previously (Esau, 1936). In the roots and shoots the first sieve elements differentiate at levels where no mature xylem is present. They show the usually chromophobic protoplasts that make the cells conspicuous among the surrounding still undifferentiated and well-stained cells (Figs. 1 and 4, a). The first sieve elements and associated cells in root tips are markedly larger than their counterparts in the leaves. Companion cells are readily identifiable in connection with the first sieve elements in the leaves (Fig. 4, c), but not, so in the roots (Fig. 1). In the subsequent growth of phloem, the first sieve elements become nonfunctional and, in the leaves, are soon crushed and completely obliterated (Fig. 6, 0). In roots the crushing is considerably delayed. Before the first conducting cells become nonfunctional, new sieve elements differentiate centripetally from the first and, in the leaves, also to the right and to the left from the first (Fig. 6, s). After a time the new elements are also obliterated. The process of elimination of old sieve elements and t’heir replacement by new ones continues till the end of
350
ESAU
FIGS. 1-6. Cross sections of roots (Figs. l-3) and leaves (Figs. 4-6) comparing healthy celery material (Figs. 1, 4, 6) wit,h that affected by the yellow leafroll virus of peach (Figs. 2, 3, 5). Details are: c, companion cell; o, obliterated cells; s, sieve element. Incipient necrosis of sieve elements, or associated cells, or both are shown in Figs. 2, 3, and 5. Magnification: X 850 (all figures).
PHLOEM
DEGENERATION
IN
INFECTED
CELERY
351
development of shoot or root. As was pointed out previously (Esau, 1936), in celery leaves the old phloem that no longer contains any conducting cells becomes the thick-walled “bundle cap” on the outer border of the vascular bundle. In plants affected by the virus, degenerative changes may be observed as soon as the first sieve elements mature. Either the sieve elements themselves or the cells next to them show incipient necrosis. The prot,oplasts of the affected cells become increasingly chromophilic and usually rather homogeneous in appearance. Figure 2 shows two chromophilic pericyclic cells above a sieve element of a root; and in Fig. 3, the just differentiated sieve element itself is undergoing necrosis. It should be pointed out here that immature sieve elements are normally more chromophilic than the mature ones; but the staining depicted in Fig. 3 is far more intense than that of normal immature sieve elements. The element in Fig. 3 was the youngest of a longitudinal series of enucleate elements. A somewhat later stage of degeneration is depicted for a leaf in Fig. 5. Here the first sieve element has undergone necrotic obliteration (0; term according to Esau, 1957), the other (s, to the left) and itt companion cell (c) have darkly stained contents. Still another sieve element (s, to the right) does not appear to be affected. As more phloem differentiates, degenerative changes spread into this new tissue involving, as before, sieve elements, companion cells, and some phloem-pnrenchyma cells. A somewhat advanced stage of degeneration is shown in Fig. 13. In the upper part of the view much obliteration (0) has occurred. One sieve element (s, to the right), not yet crushed but already affected, has dense contents. At p are phloem parenchyma cells that show the chromophilic condition. Many other sieve elements (s) display no degenerative changes. As the disease progresses more cells become affected and eventually the tissue collapses as a mass among accumulations of highly chromophilic material, probably wound gum. Such severe necrosis is prevalent in the stem (crown) region of the plant (Fig. 10). Longitudinal sections are especially useful for observing the degenerating proboplasts in affected cells. Figures 7 and 8 reveal the sharp contrast that may be observed between the thin, barely stained, vacuolate cytoplasm of a normal enucleate sieve element (Fig. 7, s) and the dense nonvacuolated but enucleate cytoplasm of an affected element (Fig. 8, s). Figure 9 shows a similar contrast between two related parenchyma cells (p), one with dense cytoplasm and apparently dead contracted nucleus (~z), the other wit,h normally vacuolated contents and an elonga-
352
ESAU
FIGS. 7-10. Longitudinal (Figs. 7-9) and transverse (Fig. 10) sections from celery petioles. Figures 7 and 8 contrast sieve elements (s) and companion cells (c) from a healthy (Fig. 7) and a diseased (Fig. 8) plant. Of the two related parenchyma cells (p), with nuclei at n, in Fig. 9, the cell t)o the left shows incipient necrosis, the cell to the right appears normal. In Fig. 10 the collapsed necrosed part of the phloem is surrounded by divided parenchyma cells. Magnification: Figs. 7-9, X 850; Fig. 10, X 550.
ted reticulate nucleus. The differential behavior of these two related cells -they appear to be recent derivatives of a procambial cell that divided longitudinally-is worthy of note. It may indicate either differences in reactions of cells to the injurious agent or an uneven distribution of the agent. Similar phenomena might be thought of as an explanation for the occurrence of apparently normal sieve elements in a degenerating tissue (Fig. 13). The homogeneous consistency of the affected protoplast is often suc-
PHLOEM
DEGENERATION
IN INFECTED
CELERY
353
FIGS. 11-13. Cross sections of petioles from healthy (Fig. 11) and diseased (Figs. 12, 13) plants. The procambium between the xylem (containing vessels, v), and the phloem with sieve elements (s) and oil duct (d) in Fig. 11 is relatively inactive; in Fig. 12 the meristem shows results of recent divisions. Figure 13 illustrates obliteration (0) of necrosed cells, incipient necrosis of parenchyma cells (p), and various conditions of sieve elements (s) and companion cells (c). Magnification: X 850 (all figures).
354
ESAU
ceeded by the appearance of denser and lighter areas and the development of discrete particles (Fig. 2, pericyclic cell to the left). When necrosis occurs, the protoplast becomes so densely stained that no details can be discerned (Fig. 8, companion cell, c). In addition to the high degree of chromaticity of cells in localized situations, the tissues of the diseased plants in general tend to become somewhat more strongly stained and have somewhat thicker walls than those of healthy plants. Hypertrophy and hyperplasia have not been observed in the early stages of degeneration of the phloem. Subsequent,ly, when collapse of necrosed cells occurs, the adjacent cells may undergo growth and division, evidently as a result of a wound-healing reaction (Fig. 10, see cells surrounding the area of necrosis). The divisions in the wound-healing tissue are rather orderly, and the resulting cells are parenchymatous. In the buckskin-diseased peach Schneider (1945) observed excessive production of phloem by the cambium of swollen veins in leaves. The celery plant parts included in this study had no secondary growth, but there was evidence that the last procambial cells remaining at the end of primary differentiation between the xylem and the phloem were stimulated to divide and to produce additional phloem elements. Figure 11 shows the condition of the procambial cells between the xylem (containing vessels, v) and the phloem (containing sieve elements, s) toward the end of growth of a vascular bundle in a healthy leaf. The procambial tissue appears rather inactive with regard to cell division, although the last-formed vessel elements (v, to the right) are not yet mature. In contrast, the tissue between the xylem and the phloem in Fig. 12, which was taken from an affected leaf, shows evidence of numerous recent divisions; and these divisions occurred next to and among mature xylem elements. The small diameters and thick walls of the last-formed vessel elements (v, above) in Fig. 12 indicate that the growth of the xylem was completed. It appears, therefore, that the meristematic activity next to the xylem is concerned not with addition to this tissue but with contributions to the phloem. The advanced degree of differentiation of the youngest xylem in Fig. 12 is also an indication of a somewhat precocious maturation of the tissue, probably a result of a stunting effect of the disease. DISCUSSION
The foregoing presentation indicates a marked similarity between symptoms induced by the yellow leafroll virus of peach-as observed in celery-and by the Green Valley buckskin virus. This similarity sup-
PHLOEM
DEGENERATION
IN
INFECTED
CELERY
355
ports the view that these two viruses are closely related (cf. Jensen, 1956). Although the information on the anatomic effect of the yellow leafroll virus on the peach is still scanty (cf. Introduction), it suggests that the same primary degenerative changes occur in this tree host as in the celery. On the basis of the anatomic evidence one may add the yellow leafroll virus of peach to the growing list of viruses that are associated with the phloem tissue. It was pointed out previously (Esau, 1957), that viruses primarily affecting the phloem tissue may be divided in two main groups with regard to the histologic symptoms in the phloem: (1) viruses inducing early hyperplasia (e.g., curly top and aster yellows viruses) and (2) viruses inducing no early hyperplasia (e.g., barley yellow dwarf and buckskin viruses). As judged by its effect upon celery, the virus of the peach yellow leafroll disease belongs to the second group. Despibe the differences between the two groups of viruses, fundamental similarities may be detected in the initiation of the cytohistologic symptoms. In both groups, the timing in the appearance of the symptoms in newly developing plant parts, namely, their initiation at levels where the first sieve elements mature, suggests a distribution through mature sieve elements. Furthermore, in both, the initial symptoms are degenerative changes in the protoplasts either of the sieve elements that appear to be concerned with the spread of the virus, or in the parenchymatous cells next to them. In plants affected with viruses of the first group, the initial change is rapidly followed by some hypertrophy and a vigorous hyperplasia. The resulting hyperplastic tissue, which later becomes necrosed, consists mainly of abnormal sieve elements. Presence of viruses of the second group does not stimulate such hyperplasia; degeneration and subsequent necrosis of sieve-element protoplasts is the dominant symptom, accompanied, more or less consistently, by degeneration and necrosis of cells associated with the sieve elements (companion cells and phloem parenchyma cells). In both groups of virus diseases hypertrophy and hyperplasia may occur in tissues other than the phloem or in the necrosed phloem as a wound-healing response. These growth changes are entirely secondary symptoms and are of limited usefulness in diagnosis of the diseases. REFERENCES
ESAU, K. (1936). Ontogeny in celery petioles.
Hilgardia
and structure of collenchyma 10, 431-476.
and of vascular
bundles
356
ESAU
K. (1944). Anatomical and cytological studies on beet mosaic. J. Agr. Research 69, 95-117. ESAU, K. (1957). Phloem degeneration in Gramineae affected by the barley yellondwarf virus. Am. J. Botany 44, 245-251. JENSEN, D. D. (1956). Insect transmission of virus between tree and herbaceous plants. Virology 2, 249-260. SASS, J. E. (1951). Botanical Microtechnique, 2nd ed., 223 pp. Iowa State College Press, Ames, Iowa. SCHNEIDER, H. (1945). Anatomy of buckskin-diseased peach and cherry. Phytopathology 36, 610635. ESAU,
6,
Phloem
348-356 (1958)
Degeneration Leafroll
in Celery Virus of
KATHERINE Department
of Botany,
Infected Peach
with
Yellow
ESAU
University of California, Accepted May 19, 1958
Davis,
California
Apium graveolens L. (celery) experimentally infected with the yellow leafroll virus of peach develops phloem degeneration of the type characterized by necrosis of sieve elements, their companion cells, and some phloem parenchyma cells. In the incipient stages of necrosis the protoplasts of affected cells assume a homogeneous appearance and stain somewhat darker than normal cells. Later, the chromaticity increases and reaches a maximum at death of the cells. No hyperplasia was observed in the early stages of disease development, but some mild stimulation of meristematic activity occurred in the procambium in the later stages. After many cells became necrosed a pronounced wound-healing reaction, characterized by enlargement and division of cells, developed about the areas of necrosis. The initiation of phloem degeneration did not occur until one or more sieve elements were differentiated in a given region. INTRODUCTION
Recently Jensen (1956) succeeded in transmitting a virus of stone fruits to celery (Apium graveolens L.) by means of a leafhopper. The celery became diseased in a relatively short time and proved to be a better source of the virus than its tree hosts. The virus used by Jensen (1956) has been recognized in the form of four strains affecting stone fruits. One of these strains, the Green Valley buckskin virus, was found by Schneider (1945) to induce sieve-tube necrosis in peach trees. Some peach tree material infected with the yellow leafroll strain of the virus, examined by the present writer for a preliminary orientation, showed necrosis of sieve tubes and associated parenchyma cells. There was also some excessive development of secondary phloem in leaf veins, a phenomenon resembling that described by Schneider (1945) for the swollen veins of leaves of peach affected by the buckskin disease. Jensen (1956) has reviewed the difficulties encountered in studies of 348
PHLOEM
DEGENERATION
IN
INFECTED
CELERY
349
host ranges of fruit tree viruses and has pointed out the value of transmission of such viruses to herbaceous plants. If such a transmission is effected, it is desirable to know whether the internal symptoms induced by a tree virus in an herbaceous plant are similar to those found in its woody hosts. The present paper deals with the anatomic symptoms induced by the yellow leafroll virus of the peach in the celery plant. MATERIALS
AND METHODS
The inoculation of the celery plants was carried out by Dr. D. D. Jensen, University of California, Berkeley, according to the method previously described by him (*Jensen, 1956). Nine infected plants showing the symptoms of the disease in various degrees of severity and three healthy control plants were available. Shoot apices, petioles, blades, and root apices were killed in chrome-acetic-formalin solution (Craf III) (Sass, 1951, p. 18), dehydrated by means of tertiary butyl alcohol (Sass, 1951, p. 27), embedded in paraffin, sectioned at 10 F thickness, and stained with hematoxylin and safranine (Esau, 1944). RESULTS
The primary internal symptom of the disease was recognized in the phloem tissue. The normal phloem is, therefore, briefly described in the following paragraphs. A detailed study of this tissue in celery has been published previously (Esau, 1936). In the roots and shoots the first sieve elements differentiate at levels where no mature xylem is present. They show the usually chromophobic protoplasts that make the cells conspicuous among the surrounding still undifferentiated and well-stained cells (Figs. 1 and 4, a). The first sieve elements and associated cells in root tips are markedly larger than their counterparts in the leaves. Companion cells are readily identifiable in connection with the first sieve elements in the leaves (Fig. 4, c), but not, so in the roots (Fig. 1). In the subsequent growth of phloem, the first sieve elements become nonfunctional and, in the leaves, are soon crushed and completely obliterated (Fig. 6, 0). In roots the crushing is considerably delayed. Before the first conducting cells become nonfunctional, new sieve elements differentiate centripetally from the first and, in the leaves, also to the right and to the left from the first (Fig. 6, s). After a time the new elements are also obliterated. The process of elimination of old sieve elements and t’heir replacement by new ones continues till the end of
350
ESAU
FIGS. 1-6. Cross sections of roots (Figs. l-3) and leaves (Figs. 4-6) comparing healthy celery material (Figs. 1, 4, 6) wit,h that affected by the yellow leafroll virus of peach (Figs. 2, 3, 5). Details are: c, companion cell; o, obliterated cells; s, sieve element. Incipient necrosis of sieve elements, or associated cells, or both are shown in Figs. 2, 3, and 5. Magnification: X 850 (all figures).
PHLOEM
DEGENERATION
IN
INFECTED
CELERY
351
development of shoot or root. As was pointed out previously (Esau, 1936), in celery leaves the old phloem that no longer contains any conducting cells becomes the thick-walled “bundle cap” on the outer border of the vascular bundle. In plants affected by the virus, degenerative changes may be observed as soon as the first sieve elements mature. Either the sieve elements themselves or the cells next to them show incipient necrosis. The prot,oplasts of the affected cells become increasingly chromophilic and usually rather homogeneous in appearance. Figure 2 shows two chromophilic pericyclic cells above a sieve element of a root; and in Fig. 3, the just differentiated sieve element itself is undergoing necrosis. It should be pointed out here that immature sieve elements are normally more chromophilic than the mature ones; but the staining depicted in Fig. 3 is far more intense than that of normal immature sieve elements. The element in Fig. 3 was the youngest of a longitudinal series of enucleate elements. A somewhat later stage of degeneration is depicted for a leaf in Fig. 5. Here the first sieve element has undergone necrotic obliteration (0; term according to Esau, 1957), the other (s, to the left) and itt companion cell (c) have darkly stained contents. Still another sieve element (s, to the right) does not appear to be affected. As more phloem differentiates, degenerative changes spread into this new tissue involving, as before, sieve elements, companion cells, and some phloem-pnrenchyma cells. A somewhat advanced stage of degeneration is shown in Fig. 13. In the upper part of the view much obliteration (0) has occurred. One sieve element (s, to the right), not yet crushed but already affected, has dense contents. At p are phloem parenchyma cells that show the chromophilic condition. Many other sieve elements (s) display no degenerative changes. As the disease progresses more cells become affected and eventually the tissue collapses as a mass among accumulations of highly chromophilic material, probably wound gum. Such severe necrosis is prevalent in the stem (crown) region of the plant (Fig. 10). Longitudinal sections are especially useful for observing the degenerating proboplasts in affected cells. Figures 7 and 8 reveal the sharp contrast that may be observed between the thin, barely stained, vacuolate cytoplasm of a normal enucleate sieve element (Fig. 7, s) and the dense nonvacuolated but enucleate cytoplasm of an affected element (Fig. 8, s). Figure 9 shows a similar contrast between two related parenchyma cells (p), one with dense cytoplasm and apparently dead contracted nucleus (~z), the other wit,h normally vacuolated contents and an elonga-
352
ESAU
FIGS. 7-10. Longitudinal (Figs. 7-9) and transverse (Fig. 10) sections from celery petioles. Figures 7 and 8 contrast sieve elements (s) and companion cells (c) from a healthy (Fig. 7) and a diseased (Fig. 8) plant. Of the two related parenchyma cells (p), with nuclei at n, in Fig. 9, the cell t)o the left shows incipient necrosis, the cell to the right appears normal. In Fig. 10 the collapsed necrosed part of the phloem is surrounded by divided parenchyma cells. Magnification: Figs. 7-9, X 850; Fig. 10, X 550.
ted reticulate nucleus. The differential behavior of these two related cells -they appear to be recent derivatives of a procambial cell that divided longitudinally-is worthy of note. It may indicate either differences in reactions of cells to the injurious agent or an uneven distribution of the agent. Similar phenomena might be thought of as an explanation for the occurrence of apparently normal sieve elements in a degenerating tissue (Fig. 13). The homogeneous consistency of the affected protoplast is often suc-
PHLOEM
DEGENERATION
IN INFECTED
CELERY
353
FIGS. 11-13. Cross sections of petioles from healthy (Fig. 11) and diseased (Figs. 12, 13) plants. The procambium between the xylem (containing vessels, v), and the phloem with sieve elements (s) and oil duct (d) in Fig. 11 is relatively inactive; in Fig. 12 the meristem shows results of recent divisions. Figure 13 illustrates obliteration (0) of necrosed cells, incipient necrosis of parenchyma cells (p), and various conditions of sieve elements (s) and companion cells (c). Magnification: X 850 (all figures).
354
ESAU
ceeded by the appearance of denser and lighter areas and the development of discrete particles (Fig. 2, pericyclic cell to the left). When necrosis occurs, the protoplast becomes so densely stained that no details can be discerned (Fig. 8, companion cell, c). In addition to the high degree of chromaticity of cells in localized situations, the tissues of the diseased plants in general tend to become somewhat more strongly stained and have somewhat thicker walls than those of healthy plants. Hypertrophy and hyperplasia have not been observed in the early stages of degeneration of the phloem. Subsequent,ly, when collapse of necrosed cells occurs, the adjacent cells may undergo growth and division, evidently as a result of a wound-healing reaction (Fig. 10, see cells surrounding the area of necrosis). The divisions in the wound-healing tissue are rather orderly, and the resulting cells are parenchymatous. In the buckskin-diseased peach Schneider (1945) observed excessive production of phloem by the cambium of swollen veins in leaves. The celery plant parts included in this study had no secondary growth, but there was evidence that the last procambial cells remaining at the end of primary differentiation between the xylem and the phloem were stimulated to divide and to produce additional phloem elements. Figure 11 shows the condition of the procambial cells between the xylem (containing vessels, v) and the phloem (containing sieve elements, s) toward the end of growth of a vascular bundle in a healthy leaf. The procambial tissue appears rather inactive with regard to cell division, although the last-formed vessel elements (v, to the right) are not yet mature. In contrast, the tissue between the xylem and the phloem in Fig. 12, which was taken from an affected leaf, shows evidence of numerous recent divisions; and these divisions occurred next to and among mature xylem elements. The small diameters and thick walls of the last-formed vessel elements (v, above) in Fig. 12 indicate that the growth of the xylem was completed. It appears, therefore, that the meristematic activity next to the xylem is concerned not with addition to this tissue but with contributions to the phloem. The advanced degree of differentiation of the youngest xylem in Fig. 12 is also an indication of a somewhat precocious maturation of the tissue, probably a result of a stunting effect of the disease. DISCUSSION
The foregoing presentation indicates a marked similarity between symptoms induced by the yellow leafroll virus of peach-as observed in celery-and by the Green Valley buckskin virus. This similarity sup-
PHLOEM
DEGENERATION
IN
INFECTED
CELERY
355
ports the view that these two viruses are closely related (cf. Jensen, 1956). Although the information on the anatomic effect of the yellow leafroll virus on the peach is still scanty (cf. Introduction), it suggests that the same primary degenerative changes occur in this tree host as in the celery. On the basis of the anatomic evidence one may add the yellow leafroll virus of peach to the growing list of viruses that are associated with the phloem tissue. It was pointed out previously (Esau, 1957), that viruses primarily affecting the phloem tissue may be divided in two main groups with regard to the histologic symptoms in the phloem: (1) viruses inducing early hyperplasia (e.g., curly top and aster yellows viruses) and (2) viruses inducing no early hyperplasia (e.g., barley yellow dwarf and buckskin viruses). As judged by its effect upon celery, the virus of the peach yellow leafroll disease belongs to the second group. Despibe the differences between the two groups of viruses, fundamental similarities may be detected in the initiation of the cytohistologic symptoms. In both groups, the timing in the appearance of the symptoms in newly developing plant parts, namely, their initiation at levels where the first sieve elements mature, suggests a distribution through mature sieve elements. Furthermore, in both, the initial symptoms are degenerative changes in the protoplasts either of the sieve elements that appear to be concerned with the spread of the virus, or in the parenchymatous cells next to them. In plants affected with viruses of the first group, the initial change is rapidly followed by some hypertrophy and a vigorous hyperplasia. The resulting hyperplastic tissue, which later becomes necrosed, consists mainly of abnormal sieve elements. Presence of viruses of the second group does not stimulate such hyperplasia; degeneration and subsequent necrosis of sieve-element protoplasts is the dominant symptom, accompanied, more or less consistently, by degeneration and necrosis of cells associated with the sieve elements (companion cells and phloem parenchyma cells). In both groups of virus diseases hypertrophy and hyperplasia may occur in tissues other than the phloem or in the necrosed phloem as a wound-healing response. These growth changes are entirely secondary symptoms and are of limited usefulness in diagnosis of the diseases. REFERENCES
ESAU, K. (1936). Ontogeny in celery petioles.
Hilgardia
and structure of collenchyma 10, 431-476.
and of vascular
bundles
356
ESAU
K. (1944). Anatomical and cytological studies on beet mosaic. J. Agr. Research 69, 95-117. ESAU, K. (1957). Phloem degeneration in Gramineae affected by the barley yellondwarf virus. Am. J. Botany 44, 245-251. JENSEN, D. D. (1956). Insect transmission of virus between tree and herbaceous plants. Virology 2, 249-260. SASS, J. E. (1951). Botanical Microtechnique, 2nd ed., 223 pp. Iowa State College Press, Ames, Iowa. SCHNEIDER, H. (1945). Anatomy of buckskin-diseased peach and cherry. Phytopathology 36, 610635. ESAU,