Page:The Oak.djvu/89

 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).