THE SEEDLING AND YOUNG PLA.NT 71 



are left (fig. 21), the thickenings of which are at length 

 not spirals or nets for the most part, but irregular pittings. 

 Moreover, they are nearly isolated. Nevertheless, the 

 inner elements can be distinguished as primary tracheal 

 elements, because, being earlier formed, they partook 

 more in what elongation occurred, and their spirals, for 

 instance, are wider apart. 



In the midrib, in proportion as the structural changes 

 go on, the bundles approach one another, the separating 

 parenchyma becoming narrower and narrower. The 

 pith consists of parenchyma, chiefly unlignified and 

 with simple pits, but as the bundles are approached 

 the cells become longer and lignified ; the rays between 

 the xylem groups are also lignified. 



Towards autumn the cells of the pith and rays fill 

 with starch ; this is nearly, but not quite, all resorbed 

 before the leaf falls. 



The termination of the bundles in the leaf consists 

 only of a few narrow spiral and reticulated cells, which 

 at last become very short and variable in shape, and of 

 a few small sieve elements and cells (see Chapter VI.). 



