Page:The Oak.djvu/60

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42
THE OAK.

next neighbor below, and if a line be drawn from the insertion of any one leaf through the points of insertion of those above, it will describe a spiral, and will eventually come to a leaf standing directly above the leaf started from. In doing this the spiral line will pass twice round the stem, and through the points of insertion of five leaves. This is shortly expressed by two fifths.

The previously homogeneous embryonic tissue in the section now shows certain patches of grayer, closer tissue, arranged round the center in a peculiar manner; these are transverse sections of the young vascular bundles—strands which at present are distinguished chiefly by the small diameter of their cells, whence the darker gray appearance.

These strands when young are called procambium strands. Their cells are distinguished from the other embryonic cells around by growing more in length and dividing less frequently across their length, and by growing less in breadth and dividing more often by longitudinal walls.

On transverse sections a little lower down there may be seen a number of elongated and curved patches of procambium, as shown in Fig. 9. On the section it will be noticed that the larger strands are so arranged that they inclose a five-angled mass of central tissue (the pith), the five comers pointing to the angles of the young stem to which the leaves are attached. At the corners or ends of the rays just referred to are in some cases two or three smaller strands.