The natural history of plants, their forms, growth, reproduction, and distribution; . aple. The stalks of every pairof opposite leaves are of equal length on the erect branch. But how entirelydifferent in respect to length are those leaf-stalks which embellish the horizontally- 420 RELATION BETWEEN POSITION AND FORM OF GREEN LEAVES. directed branches of this species. Here one of the pair always appears con-siderably longer than the other; and it is not a rare occurrence for it to bethree times as long as its neighbour, as may be seen in figure 109. And whythis striking dissimilarity? The reaso
The natural history of plants, their forms, growth, reproduction, and distribution; . aple. The stalks of every pairof opposite leaves are of equal length on the erect branch. But how entirelydifferent in respect to length are those leaf-stalks which embellish the horizontally- 420 RELATION BETWEEN POSITION AND FORM OF GREEN LEAVES. directed branches of this species. Here one of the pair always appears con-siderably longer than the other; and it is not a rare occurrence for it to bethree times as long as its neighbour, as may be seen in figure 109. And whythis striking dissimilarity? The reason is again the same as in all the previouscases. If all the leaf-stalks were to retain the same length on the horizontal twigswhich they have on the erect branches (see fig. 106), then one of the leaves ofevery alternate pair would come to be very unfavourably situated in its neighboursshadow. This detrimental condition must be prevented, and this may be effectedmost simply by the leaf-stalk increasing in length until the blade it carries isprojected beyond the area of the Fig. 110.—Leaf-mosaics of Unsyninietrical Begonia Dregei growing in front of a vertical walL 2 picus scandens, growing on a vertical wall It may be expected that alterations of direction, shortenings and elongations,similar to those just described in the case of the horizontal leafy twigs of the lowerboughs of trees, shrubs, and bushes, will be found on those plants which areattached to a steep face of rock, a vertical wall, or to the bark of an upright tree-trunk. As a matter of fact all the instances discussed here are again met with invarious climbing and twining growths, as well as in those whose stem is parallel toa vertical wall without being attached to it, as in Bhamnus pumila, and inmany begonias. But here the leaf-blades do not place themselves parallel to theground, but to that surface on which the plants in question are supported, or whichthey adjoin. In these plants anoth
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectbotany, bookyear1902
