The elements of botany for The elements of botany for beginners and for schools elementsofbotany00grayuoft Year: [1887] SECTION 7.] THEIR ARRANGEMENT. 69 185. Phyllotaxy of Alternate Leaves. Alternate leaves are distrib- uted along the stem iu an order which is uniform for each species. The arrangement in all its modifications is said to be spiral, because, if we draw a line from the insertion (i. e. the point of attachment) of one leaf to that of the next, and so on, this line will wind spirally around the stem as it rises, and in the same species will always bear the same number of leaves f
The elements of botany for The elements of botany for beginners and for schools elementsofbotany00grayuoft Year: [1887] SECTION 7.] THEIR ARRANGEMENT. 69 185. Phyllotaxy of Alternate Leaves. Alternate leaves are distrib- uted along the stem iu an order which is uniform for each species. The arrangement in all its modifications is said to be spiral, because, if we draw a line from the insertion (i. e. the point of attachment) of one leaf to that of the next, and so on, this line will wind spirally around the stem as it rises, and in the same species will always bear the same number of leaves for each turn round the stem. That is, any two successive leaves will always be separated from each other by an equal portion of the circum- ference of the stem. The distance in height between any two leaves may vary greatly, even on the same shoot, for that depends upon the length of the internodes, or spaces between the leaves; but the distance as measured around the circumference (in other words, the Angular Divergence, or angle formed by any two successive leaves) is uniformly the same. 186. Two-ranked. The greatest possible di- vergence is, of course, where the second leaf stands on the opposite side of the stem from the first, the third on the side opposite the second, and therefore over the first, and the fourth over the second. Tliis brings all the leaves into two ranks, one on one side of the stem and one on the other, and is therefore called tho. Two-ranked arrangement. It occurs in all Grasses, — in Indian Corn, for in- stance ; also, in the Basswood (Fig. 181). This is the simplest of all arrangements, and the one which most widely distributes successive leaves, but which therefore gives the fewest vertical ranks. Next is the 187. Three-ranked arrangement, — that of all Sedges, and of White Hellebore. Here the second leaf is placed one third of the way round the stem, the third leaf two thirds of the way round, the fourth leaf accordingly directly over t
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