112 The Phenomena of Morphogenesis 



trol and proceeds in a regular and orderly fashion (Fig. 5-7). Since 

 most plant organs are determinate structures, their forms are more con- 

 stant and precise than are those of the whole plant body. 



The origin of specific form in a plant or its organs may be studied in 

 embryological development but more readily in the growth of organs that 

 originate at the meristem, especially leaves, flowers, and fruits. In some 

 cases the mature form or a close approximation to it is established very 

 early, and from an examination of a tiny primordium, when its size may 

 be only a fraction of a cubic millimeter, the final shape of the organ can 

 be seen. The critical period in form determination here is evidently near 

 the beginning of development. More frequently, however, the early 

 primordium is simple, often nearly isodiametric, and the final form de- 

 velops by differential growth. 



too 



i 

 i- 

 o 



z 



Id 



-f a * 

 22 . 



z> * 

 J N 



x * 

 < 



2 



6 

 o 



10' 



10 



I I I I 



I I I Mill 

 100 



LOG MAXIMUM WIDTH 



x 2 x 10V 



Fig. 5-7. Relative growth of length to width in developing fern prothallium. (From 

 Albaum. ) 



Plant embryology in its widest sense is the record of such differential 

 growth by which the complexity of organic form is attained. Most of 

 our knowledge of the process is from verbal or pictorial descriptions, 

 but in some cases it has been analyzed more precisely. The techniques 

 of measuring allometric growth are as applicable in such cases as they 

 are in the more frequently studied ones of part-to-part analysis. Richards 

 and Kavanagh ( 1943 ) have extended the method further and show how 

 it may be applied to three-dimensional growth. If this proves generally 

 feasible, analysis of form development will become much more precise. 

 Schuepp ( 1945, 1946 ) has used the methods of allometry to supplement 

 others in a rather complex analysis of the development of leaf shape and 

 of the origin of the leaf primordium at the meristem. 



Sinnott (1936fr) applied these methods to the study of form develop- 

 ment in fruits of various races of cucurbits where form difference is due 



