Page:The Oak.djvu/132

114 are traversed by very numerous pit-canals; cells containing crystals also accompany these groups.

In consequence of the above arrangements the secondary cortex presents a more or less stratified appearance on the transverse section, the strata consisting chiefly of alternate tangential layers of hard bast and soft bast (Fig. 17); the elements of the latter also showing a decided tendency to be arranged in layers.

After the first year the young stem or branches covered with thin periderm are seen to be dotted with lenticels or cortical pores. Structures similar in every respect and subserving the same function—viz., the exchange of gases with the environment—are formed on the roots as soon as the periderm is developed.

The lenticel is a local interruption of the periderm, where the cells are loosened so as to allow air to pass between the loosened cells into the intercellular spaces between the cortical cells. Each lenticel may be described as a biconvex projecting swelling of the periderm, the swelling being caused by the increased radial diameter of the loosened cells. This is the condition during the spring and summer, but in the winter the cork-cambium is continuous across beneath the lenticel, and forms periderm in an uninterrupted sheet, to be ruptured again at the lenticel during the formation and swelling of the looser cells (complementary or packing cells) in the following spring. These loose packing-cells are at first quite similar to young cork-cells, and are developed as such, but they loosen and round off, and their