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its function, is obviously in process of development, for it has not only retained all five digits, but, what is functionally a sixth digit, has made its appearance (rs.). The phalanges are very well developed (phal.), but, on the other hand, the distal phalanges (ph.) have degenerated and are very short.

Heterocephalus (fig. 36) is a burrowing rodent, only, instead of being talpoide as is the Bathyergus, ie. instead of exhibiting a body furnished with

FIG. 36.-Heterocephalus Philippi. (After Oldfield Thomas.)

strong short legs, Heterocephalus looks more like an ordinary quadruped. In process of adaptation to an underground life, the hair, especially that on the legs, has disappeared. This is the more remarkable, as in no other case does it occur among mammals unless as an adaptation to aquatic life. Here it is in the atrophy of the roots of the hair that degeneration is manifested.

One form of hair, however, is still to be found on Heterocephalus, for there are a few bristles on the outer sides of the feet. These serve as brushes to sweep away the sand while burrowing.

SECTION II.

Modification of the organs of plants.

§ 4. Modification of homodynamic organs in the Individual.

For the same reasons as we have already pointed in the case of vertebrates, those leaves

4

which grow out of

different parts of the

main stem, and have

FIG. 37.-Growing point of Vicia varia,
showing the origin of leaves.

1, 2, 3, 4, leaves respectively older; leaf 1 is

still in the primordial condition; leaf 2 is

divided into epipodium and hypopodium (h). In leaves 3 and 4 the epipod is divided into petiole and leaflets, and the hypopod forms stipules.

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order to adapt themselves to their various functions, will best serve as examples for our present demonstration. In descriptive botanical treatises the term "leaf”" is applied without discrimination. As a

rule, however, this

term should only be applied to the true leaves, i.e. those of which the parts are fully developed throughout.1

1 The embryological development of the leaf among the Angiosperms (fig. 37) exhibits the following phases :-The leaf arises from the growing point in the form of a small bud, which grows

These foliage leaves, which are solely assimilative, are philogenetically the oldest, for the assimilative function of leaves was certainly an earlier function than any they may now exercise; but it is not uncommon to find both basilar (Niederblätter) and apical leaves (Hochblätter) as well as foliage leaves (Laubblätter) in the same plant, and even growing from the same branch. These leaves, which arise directly from the foliage leaves, have more or less completely lost their primitive function, and have assumed others. Their structure has undergone corresponding modifications which are all attended for some time without any evidences of differentiation, and constitutes what is called the primordial leaf. Next the leaf is differentiated into a proximal part, which almost surrounds the stem (the hypopod), a distal part (the epipod), and an intermediate part (the mesopod). From this point the various parts of the leaf begin to develop individually.

Little foliacious lamina (the stipules) frequently arise laterally from the hypopod, and, while still in the bud these stipules, being often much larger than the rest of the leaf, serve as a very effective protection to the young organs.

The way in which the epipod differentiates varies very much in different cases. The lamina is developed from it, and little buds are gradually formed around the point until the whole is ready to branch out. At this stage the young leaf is still rolled up within itself, and protected by stipules, when there are any.

Next, the mesopod grows into the petiole, and the growth of the petiole causes the separation of the different parts of the leaf.

The leaves of many plants are much less complicated than these. Sometimes the mesopod is missing, and the hypopod and the epipod are left in contact. Sometimes there is practically no hypopod, and in some cases the primordial leaf develops without any differentiation into hypopod and epipod. See fig. 49 (Sempervivum arachnoideum).

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by degeneration. A good demonstration of this will be to examine the leaves of the following types: Rosa rugosa, Serratula centauroides, Sagittaria sagittifolia, Lathyrus Aphaca, and Nymphaea dentata.1

1. Rosa rugosa.-In the foliage leaves (fig. 38, F) of a rose-branch, such as of Rosa rugosa, there are two

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Fig. 38.-Leaves from the same branch of Rosa rugosa.

A-E, successive basilar leaves; F, foliage leaf; G-1, successive apical leaves.

lateral stipules which arise from the hypopod and are fused with the base of the petiole. The petiole, which arises from the mesopod, has from two to six pairs of leaflets, and also a terminal leaflet—all of which are derived from the epipod.

1 In studying these types we will confine our attention to the leaves arising directly from the foliage leaves, to the exclusion of the floral leaves.

The basilar leaves (fig. 38, A to E), which are usually withered in an adult branch, have more or less completely lost their assimilative function, and merely serve as a protection to the bud. The hypopod plays an important part in this modification, but there are evidences of a partial degeneration, the epipod being reduced in size in proportion as the assimilating function is lost.1

The apical leaves also have partially lost their assimilative function, and have assumed, like the basilar leaves, a protective function which they exercise on

same branch of Serratula centauroides. A, foliage leaf; B, C, D, E, successive apical leaves;

the floral buds. This modification FIG. 39.—Leaves from the is effected differently to that of the basilar leaves, but is equally attended with evidences of degeneration. The hypopod continues to gain, as the epipod loses, in importance, but the leaflets, instead of de

F,bract of the involucre.

1 On examining the foliage leaves of a plant from above downwards, one finds successively leaves with large stipules and small leaflets (fig 38, E), then leaves with very small leaflets crowded at the end of the hypopod and which have only the basilar part of the petiole (fig. 38, D), and then leaves of which the epipod and the free part of the petiole have disappeared (fig. 38, c). Finally, at the base of the plant there are leaves in which the hypopod is markedly reduced, and which carry nothing but small stipules (fig. 38, A and B).

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