Page:The Oak.djvu/42

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the seedling and young plant.

Before proceeding to describe the further growth and development of the seedling, it will be well to examine its structure in this comparatively simple stage, in order to obtain points of view for our studies at a later period. For many reasons it is advantageous to begin with the root-system. If we cut a neat section accurately transverse to the long axis of the root, and a few millimetres behind its tip, the following parts may be discerned with the aid of a good lens, or a microscope, on the flat face of the almost colorless section. A circular area of grayish cells occupies the centre—this is called the axis cylinder of the young root (Fig. 5, a, a). Surrounding this is a wide margin of larger cells, forming a sort of sheathing cylinder to this axial one, and termed the root-cortex. The superficial layer of cells of this root-cortex has been distinguished as a special tissue, like an epidermis, and as it is the layer which alone produces the root-hairs, we may conveniently regard it as worthy of distinction as the piliferous layer (Fig. 5, e).

Similar thin sections a little nearer the tip of the