The Development of Stems and Leaves 



F. W. WENT 



THIS paper is not intended as a general review of the field of leaf 

 growth or stem growth factors other than auxin. It will be an 

 attempt to correlate a limited number of otherwise unrelated facts and 

 experimental evidence, special stress being laid on work carried out in 

 the Kerckhoff Laboratories at the California Institute of Technology in 

 Pasadena. 



Leaf Growth. — The problem of plant form is usually considered to 

 lie within the domain of the discipline of morphology. Whereas in its 

 inception this was purely descriptive, during the last half century, and 

 especially since the articles by Sachs on "Stoff und Form" (22), an 

 experimental morphology has been developed too. If we restrict our 

 considerations to leaves we can divide the analysis of their form in two 

 broad groups. 



On the one hand the shape of a leaf can be considered as a basic 

 concept, much as the atom is the basic unit for a chemist. The idealistic 

 morphology, of which Troll (30) is a recent exponent, considers form 

 as something absolute which may be modified by external conditions 

 but which essentially can not be broken down into constituent elements. 



On the other hand leaf form can be envisaged as being the resultant 

 of all factors contributing to the length, width, and thickness of the 

 organ. Such a point of view does not consider form as a category. Most 

 investigators who have to deal with form — experimental morphologists 

 and geneticists — use this approach, and it is the only possible approach 

 for the physiologist. Modern exponents of this point of view are Huxley 

 (13) and Sinnott (26). 



Experimental morphology uses the latter approach, and in explaining 

 the differences obtained as a result of particular treatments the prevailing 



