152 STRUCTURAL BOTANY 



many others, the ovules are inserted on that part 

 which corresponds to the margins of the carpels. 



The development and minute structure will be 

 dealt with below (p. 173). 



II. INTERNAL STRUCTURE OF THE VEGETATIVE 

 ORGANS 



In the Lily we shall find the same great systems of 

 tissue which we observed in the Wallflower, and their 

 general arrangement is also similar. There are, how- 

 ever, some broad differences between the two types, 

 chiefly connected with the course and development of 

 the vascular bundles. 



a. Stem 



We shall find it best to begin with the leafy flower- 

 ing stem (see Fig. 57). If we examine a transverse 

 section through an internode, the first thing that strikes 

 us is the very well defined central cylinder, limited on 

 the outside by a zone of thick-walled cells (pc in Fig. 57). 

 In the central cylinder we see the transverse sections 

 of a large number of vascular bundles (6). They do 

 not form a single ring, as in our dicotyledonous type, 

 but appear at first sight to be irregularly scattered. 

 We can, however, make out an arrangement in con- 

 centric rings, though they are not very regular. 

 Towards the middle of the stem we find the larger 

 vascular bundles forming an irregular ring (Jj). Sur- 

 rounding these is a zone of smaller bundles ( 2 ) 5 while 

 quite at the outside of the cylinder, next the thick- 

 walled tissue, we find the smallest bundles (Z> 3 ), which 



