frequently it is absent, in the lower races in

nineteen per cent. of cases examined; in the higher races in forty-two per cent.

The reduction in number and size of these teeth is due to a reduction in size of the jaw-bones, a cause which also frequently produces distortion in

a

the

GS arrangement of the other teeth. The rudiments of the wisdom teeth appear on the maxillary tuberosity and on the coronoid process; it is only after eruption that they come into normal connection with the jawbones.

FIG. 76.-Tip of an ear of Lolium perenne, with two earlets, the superior bearing two glumes, the lateral earlet with only the superior glume. In the lateral earlet the lower flower is open

2. Atrophy of the

and has two glumules; all the other superior glume.—Among

flowers are closed and exhibit only the inferior glumule.

GI, inferior glume; GS. superior glume;

plants there are few cases

gi, inferior glumule; gs, superior of atrophy as a result of lack of space. In grasses

glumule.

of the genus Lolium, the earlets are arranged in a spike, but in such a fashion that only the terminal earlet has space for both glumes (fig. 76). The

superior glumes remain; the inferior, pressed against the axis, disappear after the embryonic development of the flower.

3. Degeneration of palea and of stamens.-Lack of space is probably the immediate cause of the disappearance of paleæ in the receptacles of some composite flowers and of the posterior stamen in the flowers of some Scrophulariaceæ and Labiates.

In normal racemose inflorescences each floret grows in the axil of a reduced leaf called a bract. When the axis of the inflorescence is shortened and the florets crowded, as in the capitula of composite flowers, it frequently happens that the bracts of the florets (termed palea) disappear. This absence is most usual when the capitulum is small and the florets are large.

In Labiates and most Scrophulariaceæ, although the ancestral number of stamens was five, there are not more than four present; when only one is absent, it is the original posterior stamen which was pressed against the axis of the inflorescence..

§ 2. Atrophy from lack of use.

1. FUNCTIONAL INUTILITY.

(1) Etiolated plants and immobile limbs.-We have already quoted as instances of accidental atrophy, cases of degeneration of leaves in etio

lated plants, and of muscles in unused limbs (see fig. 75, p. 253).

FIG .77.-Nymphaea alba scwn on the mud and at different depths in it. 1, 2, 3, successive stages of the same seedling. Eau, water; limon, mud.

(2) Epicotyl and primary leaf of Nymphaea. These structures in the water-lily are good examples of normal atrophy in individuals. During germination (fig. 77) the cotyledons of the water-lily remain inside the seed, and a new organ (at right angles to them) grows vertically upwards. The lower part of this is the first internode of the stem (epicotyl), and the upper part is a primary acicular leaf. It grows upwards through the mud until the summit of the leaf reaches light. The growth of the epicotyl is then much slower, and its terminal bud begins to shoot out horizontally. The use of this growth of the epicotyl and primary leaf is to carry the bud to the light. When that purpose is achieved, these structures atrophy. A similar occurrence may be found in Sagittaria sagittifolia (fig. 40, H, I, p. 72). In that case, however, it is the hypocotyl which elongates, until light is reached, and then degenerates.

(3) Roots of Utricularia, cotyledons of parasitic plants, leaves transformed to spines in Phyllocactus crenatus.—As instances of atrophy throughout a species produced by inutility of the parts concerned, we have already mentioned the roots of Utricularia and the cotyledons of the parasitic plants Cuscuta, Orobanche, etc.

The spines of Phyllocactus crenatus produced from modified leaves are another example (fig. 78). Above the rounded base by which a branch of Phyllocactus is attached to older branches, there

is an angular region, the sides of which are prominent, and bear leaves modified into spines to serve as protecting organs, as in the similar case of Cereus. Higher up the branch the prominent sides become flat, and the spines are replaced by minute scales. This degeneration is the result of loss of utility. The Phyllocacti are epiphytes, and their situation consequently places them out of the reach of cropping animals. The spines near

FIG. 78.-Branch of Phyllocactus crenatus.

the basis of the branches are a survival from the terrestrial ancestors of Phyllocactus. The spines higher up have degenerated.

Animals offer many instances of atrophy as a result of inutility, both in individuals and in species.

(4) Atrophy of the branchial arches in mammals.-As they are no longer functional, most of the mammalian branchial arches atrophy. Three pairs alone persist, and of these it is only those parts which are useful. (5) Atrophy of ventral fins.—Instances of atrophy through uselessness in species are to be found in the ventral fins of fishes like the Pediculati, which live in the mud, or in Protopterus, which for a part of the year is completely buried in mud (fig. 19, p. 44).