118 WILLIAM J. ROBBINS 



production of para-chlorophenoxyacetic acid. We might say that this com- 

 pound and the Kalanchoe plant acted temporarily as linked genes. 



Many other kinds of abnormal growth in plants are probably the result of 

 the effect of minute amounts of specific chemical compounds. Insect galls 

 are characterized by an abnormal but specific growth pattern superimposed 

 on normal tissue by the presence of a foreign living organism. It seems very 

 likely from the observations of Boysen Jensen that the abnormal growth of 

 insect galls is caused by specific chemical compounds produced by the larvae 

 which inhabit the galls. 



It must be emphasized that growth is an extremely complex process, not 

 just a series of chemical reactions. To consider it as such is admittedly an 

 oversimplification giving no thought to the organization in which these re- 

 actions occur, or to the structural elements, physical processes, and chemical 

 reactions which must play a role. 



The concept of growth as a series of catalyzed reactions is useful and 

 stimulating, however, in considering the role of essential metabolites — 

 especially enzymes — and the action of inhibitors and minute amounts of 

 specific chemical compounds. 



HYBRID VIGOR 



Some years ago I attempted to determine whether hybrid corn contains a 

 greater quantity of substances which stimulate the early growth of Phyco- 

 myces Blakesleeanus than the inbred parents. The effect of extracts of air 

 dry grains and of partially germinated grains of the hybrid corn and its in- 

 bred parents was determined on the growth of Phycomyces in the presence of 

 thiamine (Robbins, 1940, 1941a). 



When compared on the basis of extract per grain, I found that the extracts 

 of the grains of the hybrid corn gave a greater dry weight of mycelium of 

 Phycomyces than those of either of the inbred parents (Fig. 7.2). The stimu- 

 lating material seemed to be present in both the embryo and the endosperm. 

 Since the solution in which the beneficial effects of the extracts were exhibited 

 contained sugar, asparagine, mineral salts, and thiamine, it appeared that 

 the effect was produced by unidentified growth substances. These were 

 termed for convenience, factor Z. 



After estimating the amount of factor Z present — from the effects of the 

 extracts of the corn grains on the early growth of Phycomyces in the presence 

 of thiamine — the following generalities seemed permissible. The amount 

 of factor Z increased with the time of the germination of the corn grains, at 

 least up to seventy-two hours' germination. The quantity of Z was greater 

 per endosperm than per embryo, and was greater in the grains of the hybrid 

 than in those of either parent. The amount of thiamine and its intermediates 

 in the embryo and endosperm of the grains of the hybrid and its parents 

 was not correlated with the amount of factor Z, nor did the amount of biotin 

 in the extracts appear to be correlated with the amount of factor Z. 



