CELLS AND TISSUES 



11 



The bodies of such organisms may be surprisingly elaborate, but the 

 degree of complexity which they attain is nevertheless limited. The 

 functional differentiation of regions in continuous large masses of proto- 

 plasm, which shows itself in visible structural differentiations, may be 

 considerable; but the degree of differentiation reached by the higher 

 classes of plants and animals seems to have been very largely conditioned 

 by the development of partitions between the various centers of activity 



Fig. 6. — A, Caulerpa macrodisca, a ccenocytic plant, 

 showing supporting trabeculse. 



B, section of leaf of Caulerpa prolifera, 

 iAfter Oltmanns.) 



(nuclei) and functionally differentiating regions, the regions thus set 

 apart then becoming more fully specialized than would have been possible 

 in a continuous aqueous colloidal medium (see R. S. Lillie, 1923). The 

 protoplasmic body thus attains a multicellular organization. 



Differentiation in multicellular masses is well exemplified in the 

 growing points of vascular plants (Fig. 7). In the stem tip and root tip 

 there are actively growing regions known as meristems, in which the cells 

 are in a relatively undifferentiated ''meristematic" or "embryonic" 

 condition.'" In mahy plants there is another extensive region of such 



1" For an extensive account of meristems, see Schiiepp (1926). 



