64 BOTANY 



When we examine the plumule we find it to be com- 

 posed of cells resembling those of the radicle. They are 

 at first all alike, and only slowly do differences become 

 apparent. At the apex we find them meristematic, 

 that is, each cell has the power to divide into two. A 

 little farther back they increase in size and become 

 vacuolated. If we take a longitudinal section at this. 

 d pe pi a e > we fi 11 ^ that, as in the 



root, we can distinguish 

 three regions which are 

 faintl Y indicated (Fig. 24). 

 The central strand or 

 plerome is visible, appear- 

 ing conical in shape as in 

 the root. Outside it lies 

 a periblem, and this is 

 covered by a dermatogen, 

 a layer of a single cell in 

 thickness. These two are 

 not conical, but are thrown 

 into irregularity by the 

 outgrowth of the leaves. 

 The leaves and branches 

 FIG. 24 . Growing point of stem of diff er in their origin from 



Dicotyledon, d, dermatogen ; pe, the branches of the root as 



young the y begin with the Ollt " 



growth of the periblem, 

 which pushes the dermatogen before it. The plerome 

 takes no part in their formation. As the plumule gets 

 older its elongation proceeds by the continued formation 

 of new cells and their subsequent growth. This goes on 

 for some time, and extends as a rule further back than it 

 does in the root. The growing region is a little more 

 complex in the stem than in the root, because the cells 

 do not all grow alike, those of the nodes, or places 

 .where the leaves arise, elongating scarcely at all, while 



