72 Growth 



influence of buds on the production of vascular tissue below them is 

 well known. When growth of an adventitious bud is induced in the epi- 

 dermis, for example, a vascular strand usually differentiates in the paren- 

 chymatous tissue below it which establishes connection with the vascular 

 tissue of the leaf or stem (p. 245). More direct experimental evidence on 

 this problem is available from other sources. When Camus (1944, 1949) 

 grafted a bud of endive into root callus in culture, provascular strands 

 were formed below it. Camus attributes this to the effect of auxin (and 

 possibly other substances) produced by the bud. Wetmore and Sorokin 

 (1955) and Wetmore (1956) have shown much the same effects from 

 lilac buds grafted into lilac callus in culture. Here the amount of 

 provascular tissue induced in the callus below was much increased if 

 auxin was also added. 



Wardlaw ( 1952c ) has called attention to the important effects on stelar 

 structure which are related to the strength of the meristematic stimulus 

 in the development of vascular structures. When leaf primordia in ferns 

 are removed, gaps in the vascular ring below are much reduced. In this 

 way it was possible to transform the axis of the normally dictyostelic 

 Dryopteris into a solenostelic form with a continuous ring. By further 

 reducing the size of the meristem through isolating it on a small piece of 

 tissue, even a protostelic condition was produced. The basis for such im- 

 portant morphological differences thus seems to be in the degree of de- 

 velopment of the meristem. 



In recent years the physiology of these tiny shoot apices has also begun 

 to be investigated. Growth substances are evidently synthesized there, 

 but just what these are and how they act are still not known. Ball ( 1944 ) 

 found that when auxin in paste was applied to the meristem apex of 

 Tropaeolum no changes were produced in it, presumably because of the 

 large amount of native auxin present; but below the tip hypertrophied 

 tissues, multiple leaves, and abnormal development of vascular tissue 

 appeared. The role of growth substances at the meristem involves many 

 important problems in plant physiology and morphogenesis (Chap. 

 18). 



By use of the techniques of paper chromatography, information is 

 being gained as to the biochemistry and particularly the nitrogenous 

 components of apical meristems. Some of the pioneer work here is de- 

 scribed by Steward and others (1954, 1955) and by Wetmore (1954). 

 The meristematic region is well supplied with free amino acids. The 

 basic ones, arginine and lysine, are more abundant in the tip of the 

 meristem than in tissues farther back, in the primordia, leaves, and 

 stem. A substantial beginning has been made toward a knowledge of 

 the distribution of these substances and similar ones, as well as of DNA 

 and various enzymes, throughout the meristematic region and at dif- 



