F. W. WENT 295 



have been developed which produce similar effects. There is no general 

 correlation between growth-promoting effects and effects on reduction 

 of leaf blade development (see for example, Zimmerman's paper on 

 page 176). The effect of 2,4-dichlorophenoxyacetic acid on tomato or 

 cotton leaves is much like that of shoestring virus or the genes discussed 

 above. Since all these widely different agents cause the same final 

 results, it must be concluded that they all affect the same basic, and 

 probably simple, process, like the phyllocaline-caulocaline balance sug- 

 gested in the previous paragraphs. 



Taxonomic Evidence. — In a large number of genera, species are dis- 

 tinguished by leaf characteristics, and again here narrower or broader 

 leaves or laciniate and hardly incised leaves characterize different species. 

 Edgar Anderson's analysis (i) is an example of how taxonomists and mor- 

 phologists move hand in hand towards a physiological explanation. He 

 made a comparison between the closely related species Nicotiana Langs- 

 dorffii and N. alata and found them to differ in 1 1 genetic coefficients. 

 Among these the leaf-vein angle was one, a sharp angle being correlated 

 with narrow leaves and narrow corolla lobes. 



In Sidalcea the basal leaves are crenate or crenately incised. Most 

 species [S. malvaejiora, S. oregana) have the upper leaves palmately 

 twice cleft into linear divisions. But in other species the upper leaves 

 are not parted or divided. In Ranunculus or Delphinium some species 

 are also characterized by leaves which are divided into narrow segments 

 whereas other species have almost entire leaves. 



Teratological Evidence. — In any book on teratology a host of examples 

 can be found in which a normally broad-leaved plant bears exceptional 

 narrow leaves, or where palmately divided leaves are found on otherwise 

 entire-leaved plants.* 



This enumeration has stressed the differential effects of light, nutri- 

 tion, viruses, genes, growth substances, normal and abnormal develop- 

 ment on caulocaline and phyllocaline production or action. Yet this does 

 not mean that leaf growth is that simple. When we know more about the 

 factors controlling leaf growth we undoubtedly will find that more 

 are involved. In the genetic analysis more than two sets of gene pairs 

 were necessary to account for the observed differences in leaf form. With 

 only two sets of leaf growth factors involved, leaf form would be much 

 less variable. Yet it seems that many of the most common variations in 



*M. T. Masters, Pflatizen-Teratologie (Leipzig, 1886), 482, 483, 516, 517, 518. 



