ITS GENERAL MORPHOLOGY. 231 



4*25. In vegetation no new organs are introduced to fulfil any 

 particular condition, but the common elements, the root, stem, and 

 leaves, are developed in peculiar and fitting forms to subserve 

 each special purpose. Thus, the same organ which constitutes 

 the stem of an herb, or the trunk of a tree, we recognize in the 

 trailing vine, or the twiner, spirally climbing other stems, in the 

 straw of Wheat and other Grasses, in the columnar trunk of the 

 Palm, in the flattened and jointed Opuntia, or Prickly Pear, arid in 

 the rounded, lump-like body of the Melon-Cactus. So, also, 

 branches harden into spines in the Thorn, or, by an opposite 

 change, become flexible and attenuated tendrils in the Vine, and 

 runners in the Strawberry; or, when developed under ground, 

 they assume the aspect of creeping roots, and sometimes form 

 thickened rootstocks, as in the Calamus, or tubers, as in the Po- 

 tato. But the type is readily seen through these disguises. They 

 are all mere modifications of the stem. The leaves, as we have 

 already seen, appear under a still greater variety of forms, some 

 of them as widely different from the common type of foliage as 

 can be imagined ; such, for example, as the thickened and obese 

 leaves of the Mesembryanthemums ; the intense scarlet or crimson 

 floral leaves of the Euchroma, or Painted Cup, of the Poinsettia 

 of our conservatories, and of several Mexican Sages ; the tendrils 

 of the Pea tribe ; the pitchers of Sarracenia (Fig. 223), and 

 also those of Nepenthes (Fig. 225), which are leaf, tendril, and 

 pitcher combined. The leaves also appear under very different 

 aspects in the same individual plant, according to the purposes 

 they are intended to subserve. The first pair of leaves, or cotyle- 

 dons, when gorged with nutritive matter for the supply of the ear- 

 liest wants of the embryo plant, as in the Bean and Almond (Fig. 

 97), would seem to be peculiar organs. But when they have dis- 

 charged this special office in germination, by yielding to the young, 

 plant the store of nourishment with which they are laden, they 

 throw off their disguise, and assume, with more or less distinct- 

 ness, the color and appearance of ordinary foliage ; while in other 

 cases, as in the Convolvulus, &c., they are green and foliaceous 

 from the first. As the stem elongates, the successive leaves vary 

 in form or size, according to the varying vigor of vegetation. In 

 our trees, we trace the last leaves of the season into bud-scales ; 

 and in the returning spring we may often observe the innermost 

 scales of the expanding leaf-buds to resume, the first perhaps im- 



