NATURE OF PLANTS 



43 



The casting off of the leaves reduces the' area of the plant 

 that is exposed to the unfavorable conditions to a minimum. 

 This reduction of the surface is effected in one way or another 

 by a great variety of plants. Much of oiir spring vegetation 

 is possible by reason of it. The spring beauties, anemones, fawn 

 lilies, iack-in-the-pulpits, etc., practically complete their growth 



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Fig. 28. Leaf fall: A, branch of horse-chestnut showing scar formed by 

 the fall of the leaf. The dots on the scar show the position of the vascular 

 bundles that are finally broken by the weight of the leaf. At the left the 

 base of the petiole is shown. B, diagram of a section through a twig of hickory 

 — s, separating layer at base of petiole; », vascular bundles. C, enlarged 

 view of the separating layer — c, cork cells that heal the wound caused by the 

 fall of the leaf. The granular cells are the outer region of the separating 

 layer, and they are beginning to break down as seen in the upper part of the 

 figure, at x, thus causing the fall of the leaf; st, cells of the stem containing 

 starch. 



before the larger summer forms appear. During this short period 

 sufficient food is manufactured by the leaves to mature the seeds 

 and fill the storage organs in the underground bulbs and stems. 



