THE SEEDLING AND YOUNG PLANT 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 proporticn as the structural changes 
go on, the bundles approach one another, the separating 
parenchyma becoming narrower and narrower. ‘Lhe 
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.). 
