THE WALLFLOWER 85 



the leaf continuing to develop long after the apex is 

 fully formed. 



We see that in each leaf-trace bundle that part 

 which belongs to the stem is formed from the plerome, 

 while that which belongs to the leaf arises from the 

 periblem. The part at the node where the vascular 

 bundle bends out towards the leaf is generally the 

 first to be formed. From this point the differentiation 

 of the vascular tissue proceeds upwards into the leaf 

 and downwards into the stem. 



When a branch arises in the axil of a leaf, it is 

 formed, like the leaf itself, entirely from the more 

 external tissues. The branch is developed a little later 

 than the leaf, and rather more layers of the periblem 

 take part in forming it. But here again the plerome 

 is not concerned in the process. 



(b) The growing point and branching of the root. 

 The root, like the shoot, forms its new tissues at 

 the apex of each branch, but as the structures to 

 be produced are different it is evident that the 

 meristem which produces them must be different 

 also. In roots there are no leaves to be formed, 

 and, on the other hand, the root-cap has to be pro- 

 vided for. Hence arise the chief differences between 

 the growing points of shoots and roots. It is 

 obvious that we must here seek the meristem, not 

 at the extreme end of the root, but inside the root- 

 cap. 



In a root like that of the Wallflower the meristem 

 of the apex consists of three layers (see Fig. 33). 

 The innermost forms the plerome, which here, as in 



