THE SEEDLING AND YOUNG PLANT. 71 



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 nar- 

 rower. The pith consists of parenchyma, chiefly un- 

 lignified 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). 



