264 The Phenomena of Morphogenesis 



the black henbane, has an annual variety and a biennial one, the annual 

 forming a flower stalk and flowers in a single season but the biennial re- 

 maining in the rosette stage for the first year unless experimentally ex- 

 posed to low temperature for some time. This difference is due to a 

 single gene, with the biennial character almost completely dominant. 

 Melchers ( 1937 ) grafted a scion from the annual variety ( and also scions 

 from Petunia, Nicotiana, and other related annual plants) into the 

 rosettes of the biennial variety in its first year and by this means induced 

 the biennial to flower in the same season. Melchers attributes this to the 

 passage across the graft union of a nonspecific flower-forming substance. 

 The genetic difference between the annual and the biennial varieties 

 seemed to be due to the ability of the former to produce a flower-form- 

 ing hormone without previous exposure to low winter temperature. 

 Melchers later (1938) found that if a short-day variety of tobacco, 

 grown under long-day conditions and thus unable to flower, was grafted 

 into biennial Hyoscyamus in its first season, the latter soon produced 

 flowers. The gene in Hijoscijamus may thus control the ability to respond 

 to the flower-inducing substance rather than to form it. 



In Petunia nyctaginiflora von Wettstein and Pirschle (1938) found a 

 gene d which differed from the normal D in producing plants that are 

 smaller and have fewer branches, smaller and more rounded leaves, and 

 a marked chlorophyll deficiency. Scions of dd grafted on DD stocks had 

 slightly larger leaves and stems and more branches but were not much 

 different from ungrafted dd. Scions of DD on dd showed general reduc- 

 tion in size and a chlorophyll deficiency, which was greatest next the 

 graft union and decreased in intensity above this. The authors believed 

 that a substance was produced in the mutant that passes into normal 

 scions and there either inhibits chlorophyll formation or causes chloro- 

 phyll degradation. Pirschle later ( 1939 ) presented evidence that dd lacks 

 a hormone, present in DD, that stimulates growth in size but does not 

 affect the shape of leaves or flowers. Objection may be raised that these 

 are nutritional and not hormonal effects. 



When tobacco and tomato are grafted with the dd mutant of Petunia, 

 their leaves show chlorophyll deficiency when the dd plant is used as a 

 stock and to a lesser degree where it is used as a scion (Pirschle, 1940). 

 The gene-produced d substance is clearly not species-specific. The 

 possibility cannot be disregarded that the supposed d mutant is actually 

 a virus infection, although its clear genetic segregation from D would sug- 

 gest that it is not. 



The single-gene mutants nana of Antirrhinum siculum and sterilis of 

 Solanum lycopersicon were produced by radium irradiations and have 

 been studied by Stein (1939). The former has a single unbranched 

 main stalk (unlike A. siculum); is flowerless; and has larger, thicker, 



