294 PLANT GROWTH SUBSTANCES 



by one or another virus. The effects of the viruses are ahvays growth 

 retardations of any of these processes, suggesting that the viruses act 

 by destroying or inactivating specific growth factors or by decreasing 

 their production. Any combination of these effects may be found in 

 various virus diseases. Doubtless a closer analysis of the virus effects 

 would reveal many more specifically affected growth factors, so that 

 observations in this field may give many clues for the existence of 

 further specific growth factors. 



Genetical EwV/e/zc^.— Practically the same disturbances in leaf form 

 and size which result from virus attack can be found as hereditary 

 malformations. Both in tobacco (deformis, see 12) and in tomato (wiry, 

 see 17) forms with linear leaves are known in which the lack of mesophyll 

 is due to single recessive genes, and which look remarkably like shoe- 

 string-virus diseased plants. Since all these abnormal leaves look much 

 alike, it is logical to assume that all the agents affect the same basic 

 growth process, causing the disappearance of phyllocaline. In other 

 pinnate leaves, such as Antirrhinum (23), genes cause narrowing of leaves, 

 whereas in palmate leaves a similar gene gives rise to laciniate forms 

 (for example, Chelidonium) . 



Sirks (27) has investigated the inheritance of leaf size in eight varieties 

 of Vicia Faba, measuring both width and length of individual leaflets. 

 He interprets his results by assuming a factor (G) for general leaf growth 

 (with various allelomorphs), which acts in conjunction with additional 

 factors for width (W) and length (B + T). Thus he definitely considers 

 leaf growth as caused by independent factors for growth in width and 



in length. 



Such examples can be augmented by citing the work of de Winton 

 and Haldane (41) on Primula, in which 5 or 6 leaf form characteristics 

 were recognized, or the work of Shull (25) on Capsella Bursa- Pa starts, 

 or the many investigations on cotton (see, for example, Stephens, 29). 



It is interesting to note that mutations which seem to exert their 

 effect by decreasing the amount of special hormones are common to a 

 number of plants. In addition to the phyllocaline-destroying genes could 

 be cited the auxin-destroying genes, which cause dwarfing and often 

 excessive branching. 



Morphogenetic Effects of Certain Growth Substances.— Zimmerman (43) 

 has described the effect which many substances with an auxin-like 

 structure have on leaf development. Since then many other chemicals 



