146

SIMPLER VARIETIES OF FORM.

firm texture, until its softer portion be in a state of decay; if then the latter be washed away by carefully directing a small stream of water against it, the skeleton will be left. Not only do leaves contain such a skeleton; but the leafy parts of flowers, and even the skins of such soft fruits as the cherry.

234. Now, with the same distribution of the veins of the leaf, many curious varieties of structure may be produced by a difference in the degree in which the space between them is filled up. One of the simplest of these is where holes are left in the blade of the leaf, in consequence of a deficiency of the fleshy portion. Some plants are particularly liable to this irregularity; which does not exist, however, where they are well supplied with nourishment. A similar, but much more curious variety exists in an aquatic plant of Madagascar, in which the fleshy cellular tissue or

parenchyma is so little developed between the veins, that the living leaf much resembles in its form one of the skeletons just described.

235. It is by no means uncommon to see the edges of leaves more or less deeply indented, according to the amount of nutriment which the plant is receiving; the distribution of the veins and the general outline of the leaf remaining the same throughout. Thus the Cochlearia, or Horseradish, has the edges of its leaves nearly smooth, when growing in a sufficiently

the same venation; a, Water, Crowfoot; rich soil; but if starved, b, Jatropha; c, Passion-flower; d, Sterculia

platanifolia; f, Dichondra; g, Asarabacca. the blade will be divided

DEFICIENCY OF FLESHY STRUCTURE.

147

into separate strips like the teeth of a comb, from the deficiency of flesh to fill up the spaces between the veins. In the accompanying figure are represented the outlines of different leaves having the same general distribution of the veins, but a different proportion of the fleshy substances between them.

236. In some plants in which the framework of the leaves is very strong, the ends of this project from the edges of the leaf, when the latter is stunted in its development, forming sharp prickles. This is the case in the Holly; the prickles on the leaves of which will be at once seen, if examined, to be simply the dried and projecting terminations of the veins. On looking at any fullgrown Holly, considerable variety will be noticed in the degree in which the leaves have this prickly character; and in general it is seen that the lower ones are the most stunted and rough, whilst the upper ones have the parenchyma of the leaf so much developed as to include these extremities, and thus to render the edges of the leaf quite smooth. Sometimes it has been observed that a Holly, growing in a very luxuriant soil, has had all its leaves in this manner metamorphosed, so as entirely to lose the peculiar aspect of the tree. This is one of the modes in which the repulsive character of some plants is softened down by cultivation. In the Cactus tribe, it would seem as if all the nourishment which would naturally form leaves is bestowed upon the stems themselves; and thus the tufts of prickles already noticed are the only indications of their place. These prickles are the woody veins; which are sometimes seen, in specimens grown in hot-houses in this country, to be converted into true though very minute leaves; in consequence, probably, of the greater supply of nourishment they receive under such circumstances, than in the dry and sterile situations they frequent in their native climes.

237. The division of leaves into leaflets may be regarded as taking place upon the same general principle. When a single series of leaflets arises from the midrib, the leaf is said to be pinnate, or winged. But sometimes, instead of leaflets arising from the midrib, we find secondary veins, from which, as from smaller midribs, secondary leaflets arise. Such are called bi-pinnate leaves.

148

STRUCTURE OF COMPOUND LEAVES.

The division may go yet further; and the secondary veins may give off their branches before any leafy parts appear on them, and

these are of course much smaller and at the same time more numerous. A leaf in which this is the case is termed tri-pinnate. Such forms are generally peculiar to different kinds of plants; but there are some in which we find them strangely intermixed, so as to display their real origin and character.

238. Such an example is afforded by the Gleditsia or Honeylocust tree of North America, known to English gardeners by the name of the three-thorned Acacia. As in other Acacias, the leaves are compound; but the division often proceeds to such different degrees in different parts of the same leaf, that it is difficult to say whether it is to be considered pinnate, bi-pinnate, or tri-pinnate.

Of such a leaf, in the accompanying figure, the central stem is the midrib ; and from it proceed the secondary veins on each side. The first pair of these bears on one side a leaflet which shows indentations marking a tendency to subdivision; and, on the other side a series of secondary leaflets, formed by the complete subdivision of the first. The second pair of veins bears on one side a series of secondary leaf

lets nearly similar; but two of these are seen to have again subdivided into more minute leaflets; the distribution of the veins in which, however, precisely corresponds with that of the larger ones, so that a skeleton of the whole would exhibit little difference in its several parts.. On the other side a portion of another series of secondary leaflets is seen; but towards the extremity they merge again into a larger leaflet. Below these again, we have a complete

SIZE OF LEAVES.-BUDS FROM EDGES.

149

pair of larger leaflets. If the whole of the leaf had been formed on this last plan, it would have been simply pinnate. If on the plan of the lowest division, in which there is a complete series of secondary leaflets on each side, the leaf would have been bi-pinnate. And if the whole leaf had been constructed upon the plan of the minutelysubdivided portion of the second division, it would have been tri-pinnale.

239. These are some of the most interesting varieties in the form of leaves, depending upon the degree in which the parenchyma or cellular flesh is supplied to fill up the interspaces between the veins. Of those which depend upon the various distribution of the veins themselves it is not intended here to speak; since every plant furnishes materials for observation of these differences. In regard to the size of leaves, it may here be mentioned that, whilst in some species they are nearly microscopic, in others, especially of the Palm tribe, single leaves attain the length of from 30 to 40 feet.

240. There are some leaves possessed of the power of developing buds from their edges, a fact which will hereafter be shown to be important. One of these is the Bog-Orchis (Malaxis paludosa) of English marshes; in which these buds may be distinctly seen, though the whole plant is very small. A better example,

however, is the Bryophyllum calycinum, which is a species inhabiting tropical climates, and known as the air-plant or leaf-plant, from the circumstance of its having no true stem or roots, but maintaining its life, and even growing and flowering, whilst hung up in a damp and warm atmosphere, without the contact of soil to any part of it.

Fig. 40. Leaf of Bryophyllum calycinum, The little buds which debearing buds at its edges.

velope themselves at the edges perfect plants before separating

150

CHANNELS AND CAVITIES FOR FLUID.

themselves from the parent; but, when they have once formed their own leaves and root-fibres, they are but little connected with it, and may be detached without injury.

241. The usual form of leaves is often remarkably changed; and many of the varieties produced in different plants seem to have for their object to collect water from the atmosphere and convey it to the roots. The large expanded leaves of the Arum tribe, for example, have a deep channel down the midrib; and this is continued along the petiole, so that the water collected by the leaf is conveyed to the point of the stem from which it springs. In the common Teazel (Dipsacus) of our own fields, and the Tillandsia, or Wild Pine of South America, there are hollows capable of holding a considerable amount of water at the point of union of the leaf-stalk with the stem.

242. But the most curious contrivances of this kind are those known as pitchers. The plants furnished with these curious receptacles are termed Pitcher-plants, and several kinds of them are known. In the Sarracenia, which is a native of Canada, these

pitchers may be distinctly seen to be

bformed by the very deep channeling of

Fig. 41. Different kinds of Pitchers; a, pitcher of Sarracenia; b, pitcher of Nepenthes; c, pitcher of Cephalotus.

leaves and leafstalks, the edges of which fold towards and meet one another, so as to form a complete vase, the mouth of which is guarded by a sort of hood formed by the top of the leaf. In the Nepenthes or Chinese Pitcherplant, the pitcher is of more complex