494 



PLANT GROWTH AND PLANT COMMUNITIES 



Figure 2. The apex as in Figure 1, with an arbitrary and formahzed indica- 

 tion of the physiological fields associated with the shoot and leaf apices. 

 Ii is the first position which, in the course of growth, will lie outside adjacent 

 inhibitional fields, and Zg will be the next to do so. 



ters of special metabolism in the basiscopic margin of the apical meri- 

 stem— is of special importance. A growth center is a multicellular locus 

 of characteristic size and shape. The leaf primordium into which it 

 develops may remain relatively localized and discrete; it may become 

 more or less extended tangentially round the meristem; or it may soon 

 cease its active growth. The evidence indicates that the growth-center 

 concept has an apt application to floral morphogenesis. 



Two further points are specially worthy of mention. Firstly, while 

 all the leaf growth centers of a species are probably closely comparable 

 metabolically, they are not necessarily identical— hence some of the 

 differences in leaf -primordium morphology in species showing hetero- 

 blastic development. Moreover, as Foster (1929, 1935) and Schuepp 

 ( 1929 ) have shown, for particular species, important differences in the 

 distribution of growth in the developing growth centers can be de- 

 tected from the outset, these resulting in the conspicuous differences 

 seen, for example, in cataphylls as compared with foliage leaves. Sec- 

 ondly, the writer (Wardlaw, 1949, 1950, 1957a) has shown that the 

 development of growth centers and very young leaf primordia can be 

 modified by surgical and chemical treatments. The latter are of special 



