ated; the blade is very thin, being composed of a few layers of cells, and there are no stomata.

The basilar leaves and the apical leaves frequently lose either partially or entirely the assimilative function which they previously exercised. Some of these leaves serve as a protection to the buds of both leaves and flowers. Others are adapted as protection from rain, and others have undergone considerable modifications owing to their existence either under water or under ground.

In all these instances of modification it can be seen that some degeneration has invariably attended each change which has taken place.

§ 5.-Modification of organs which are homologous in individuals of different species.

Having investigated the modifications of leaves in the individual, we will now give our exclusive attention to the various modifications undergone by the foliage leaves; we shall see among the various species we examine that degeneration has played a part in each instance of modification.

Of these adaptations the following are the most characteristic: adaptation to climbing, to a carnivorous diet, to aquatic life, to defence against ants, against drought, and against herbivorous animals.

1. Adaptation to climbing. Climbing plants attach themselves by means of tendrils to the nearest support within their reach. These tendrils,

which are thread-like and sensitive to contact, are

FIG. 43.-Cobaea scandens.

A, B, seedlings; the leaves of the second end in tendrils; C, young leaf of adult plant.

modifications of either stems, leaves, or roots, the

latter being the most rare. We will confine our attention at present to those adapted from leaves.

In Cobaea scandens the first two leaves of the seedling usually terminate in a leaflet (fig. 43, A); the leaves which come after are much finer and are modified into tendrils (fig. 43, c).

This modification of leaflets into tendrils, which entails the almost com

FIG. 44. Basal portion of a young plant of Vicia Pyrenaica.

plete disappearance of the original assimilative func

tion of the leaflet, is by no means uncommon. Vicia Pyrenaica (fig. 44) furnishes a good example of this change, of which the degeneration of the blade is the necessary consequence.

In Cucumis sativus (the cucumber) some of the leaves are the shape of ordinary assimilative leaves, while others are entirely modified into tendrils; in the case of the latter the blade has completely degenerated.

2. Adaptation to carnivorous nutrition.--Some plants, instead of obtaining nutriment exclusively from minerals and carbon dioxide, are capable of assimilating animal matter from insects and other small organisms, which they capture by means of a special function exercised by the leaves.

(a) Utricularia.—In the aquatic Utricularia the leaves are minutely sub-divided like many other submerged plants. The leaves of Utricularia vulgaris exhibit leaves of this type. Some of their ramifications carry pouches, which serve for the capture of minute organisms. These pouches are formed by the modification of part of the blade, and have almost completely lost their chlorophyll.

Utricularia intermedia (fig. 45) exhibits two kinds of branches; the one kind stretches out horizontally and carries green leaves; the others are devoid of chlorophyll, and the leaves are purely carnivorous.

(b) Nepenthe.-The distal extremity of the leaf

terminates in a receptacle of complicated structure which is adapted for the capture of carnivorous nutriment. Degeneration has attended this change, for the modified portion of the leaf is almost

FIG. 45.-Utricularia intermedia (after Goebel, Pflanzenbiologische Schilder

ungen, vol. ii., p. 135).

The plant has grown from a winter bud of which the remains are visible to the left. The horizontal, assimilating branch bears two pendulous twigs with carnivorous urns.

entirely without chlorophyll. A carnivorous diet, however, for these plants, is only supplementary to the typical plant nutrition, and therefore the assimilative function remains active.

F

In Utricularia this function is exercised by the leaves (as in U. intermedia), or by certain parts of the leaves (U. vulgaris). In Nepenthe, however, another and different assimilative organ is exhibited. This consists of two lateral herbaceous growths, which are not formed from the blade, but arise from the petiole. In between the receptacle and the enlarged portion of the petiole is a part in which the petiole has assumed the function of a tendril. While the receptacle is in process of formation, this tendril twines itself round a support in order to obtain the additional strength that will be required later on to uphold the receptacle when full of digestive juice.

(c) Drosera.-The leaves of the sundews (Drosera rotundifolia, D. longifolia, etc.) are furnished with a great number of emergences, each of which terminates in a digestive gland. There is a large drop of sticky fluid at the end of each gland, and these 'drops, which sparkle in the sunlight, have given the plant its name (Ros solis sundew) (fig. 46).

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These emergences contain very little chlorophyll.

"A plant of Drosera, with the edges of its leaves curled inwards so as to form a temporary stomach, with the glands of the closely-inflected tentacles pouring forth their acid secretion, which dissolves animal matter afterwards to be absorbed, may be said to feed like an animal."

-Darwin, Insectivorous Plants, p. 18.

This adaptation to a carnivorous diet has not