THE ANGIOSPERMAE : STEMS 



851 



corpus (Fig. 840). Let us emphasize that these names are not substitutes 

 for the three germinal layers of Hanstein which were described in the root. 

 They are not morphologically determined but are distinguished simply by 

 their mode of growth. Little is known as yet about the comparative organisa- 

 tion of the apex in different Angiosperms, but it is evident that there is great 

 variability, and that the tunica, for example, may be from one to four layers 

 deep. There is evidence that the various cell layers of the tunica and the 



„*i#' 



^ 



•^' 



Om 





" » i< »i >l »ff 



Tunica 

 Corpus 

 Leaf initial 



Rib meristem 



Q *"■ 



I 





Procambium 

 Axillary bud 



Fig. 840. — Solatium tuherostim. Longitudinal 

 section of the apical meristem showing tunica 

 and corpus. 



cell mass of the corpus have a good deal of physiological independence. They 

 seem to arise from a small number of separate initial cells, and the contribution 

 which the different layers make to the permanent tissue is probably very 

 variable. Certainly the outermost layer produces the epidermis and is 

 directly continuous with it. The whole plant thus lies enclosed in a cell 

 layer which can be traced back to the first periclinal divisions of the embryo, 

 and its outer surface wall is in fact the enormously extended wall of the 

 original oosphere, w-hich surrounds the plant like an amnion. The leaf 

 primordia are probably in most cases the product of the tunica layers only, 

 but this cannot be definitely asserted. 



The origin of these leaf primordia seems to be largely the mechanical 

 result of combining area-growth in the tunica and volume-growth in the 

 corpus. The balance between the two is maintained for a period by re- 

 adjustment of the cells in their relative positions, which probably includes 

 some sliding of cells against each other and changes in their shape, but 

 eventually the increase of surface leads to the appearance of a fold on the 



