MODIFICATIONS OF LEAVES AND STIPULES. 



415 



form are due to the joint effect of vigour of growth and assimilation. 

 At first both are feeble and equal ; but during the period of their maxima 

 the growth, as seen in the size of the blade, is not quite supported by the 

 assimilative process ; hence the blade is imperfect. As, however, the 

 season declines both become again equalized, and the small complete leaf 

 is formed like the first. Such, having become a fixed and hereditary 

 habit, may be seen on any bush of this species. 



Winter-buds. — With regard to the structure of the winter-bud the 

 protecting scales are often simply arrested petioles or their basal portions. 

 This is easy to be seen in the case of the horsechestnut, or currant, if the 

 expanding bud in spring-time be examined. The inner parts consist of 

 the delicate rudiments of next year's stem and foliage, with or without 

 a flower-bud, the innermost scales being sometimes hairy or woolly, as 

 an additional protection against cold. The scales gradually fall off as the 

 bud expands and the shoot begins to grow. 



In many trees the scales are not petiolar but stipular, as in oak, 

 lime, &c. ; but in the Bosaceae the stipules sometimes combine with the 

 petiole to make the scales, as in the whitebeam (fig; 84). More rarely 

 the blade of a sessile leaf acts as a protecting scale, but this is the 



st. D st. 



Fig. 84. — Stipules of Whitebeam combining with Petiole to form 

 Bud Scales ; (st) Stipules, (p) Base of Petiole, (I) Leaf Blade. 



case with the lilac (fig. 85). The blade of the leaf of the tulip-tree also 

 acts as a protection by covering the bud. 



In the case of bulbs the protecting scales are often merely the 

 sheathing bases of the leaves, the upper part being arrested. In Allium 

 Crameri, which has to endure a temperature of upwards of 120° Fahr. in 

 the desert sand near Cairo, the outermost scales, though of cellular tissue 

 only, have almost a woody texture. 



There is still another use which the base of the petiole may sustain — 

 viz. as a storehouse of nourishment and water. This is the case with 

 bulbs, and in the wood-sorrel it will be found that the petiole articulates 

 above the point of insertion, the swollen portion remaining containing 

 an abundance of starch. 



Stipules. — The main use of these organs to the plant, when associated 

 with a fully developed leaf, is to protect the new bud in its axil, especially 

 when the petiole is not dilated at the base, as in the garden-pea. In this 

 and many others the stipules are green, and fulfil the functions of a leaf, 

 being enlarged in compensation for the loss of some of the leaflets now 

 utilized as tendrils. In Lathy rus Aphaca (fig. 8G) the leaf is often entirely 

 suppressed. In the Galiums, as stated, the stipules are precisely like 

 the leaves ; hence they form together whorls, which suggested the 



