624 PLANT GROWTH AND PLANT COMMUNITIES 



fects of viruses, such as aerial tubers on potato plants, phyllody in 

 flowers, big buds, adventitious roots and shoots, etc. 



The vein-clearing symptom so common to virus diseases has been 

 shown in the case of beet yellows ( Lackey, 1954 ) and beet curly top 

 (Esau, 1956) to be due to hypertrophy of cells adjacent to the veins. 

 These cells obliterate intercellular spaces and, since they contain little 

 chlorophyll, appear more translucent than normal tissue. 



Some viruses appear to be located in or restricted to xylem tissue. 

 Houston et al. ( 1947 ) showed that vectors of the Pierce's disease virus 

 could eflFect transmission, that the virus could multiply only when the 

 vectors fed on xylem tissue, and that the virus was probably trans- 

 ported upward in the tracheary elements of the stem. Anatomical ef- 

 fects of this disease are also related to the xylem (Esau, 1948b). Deposits 

 of gums occur in vessels and other xylem cells, and an excessive de- 

 velopment of tyloses occurs in the wood. Vessel occlusions occur be- 

 fore the appearance of external symptoms and first in the inoculated 

 leaf. The phony virus of peach shows a similar pathologic effect. 



Virus-induced overgrowths in plants are perhaps best illustrated 

 by the galls of the Fiji disease of sugar cane, the tumors of the wound- 

 tumor disease of clover, the swollen stems of cacao infected with the 

 swollen-shoot virus, and enations produced by certain viruses. The 

 galls of the Fiji disease of sugar cane consist of elongated swellings on 

 the undersurface of leaves. Their restriction to the undersurface of 

 leaves is due to the fact that they are of phloem origin and the phloem 

 side of the vascular bundle is to the outside ( Kunkel, 1924 ) . The virus 

 stimulates phloem cells to proliferate and enlarge to produce the gall. 

 In late development, cells in the outer layer of the gall become highly 

 lignified and produce a distinct woody covering. 



The wound-tumor disease is a striking example of overgrowths due 

 to hypertrophy and hyperplasia (Black, 1945; Kelley and Black, 1949; 

 Lee, 1955). The wound-tumor virus affects a number of host species, 

 causing various types of overgrowths, such as leaf enations and root and 

 stem tumors (Figure 4). The tumors originate in the primary phloem 

 fibers of stems or from the pericycle of roots. Both hyperplasia and 

 hypertrophy are a part of tumor development, the former being more 

 important in the later stages of development. Many microscopic tumor 

 initials occur in stems of infected clover that do not develop into 

 macroscopic overgrowths ( Lee, 1955 ) . Black and Lee ( 1957 ) showed 

 that tumor development from such initials could be induced by use of 

 growth-promoting substances such as alpha-naphthalene acetic acid. 



The swollen-shoot virus induces conspicuous areas of enlargement 

 of the stem and root of the cacao plant. This effect results primarily 

 from hyperplasia and hypertrophy of both phloem and xylem, the 



