of this country; the other having no proper net-work, but a set of parallel or diverging veins running from the base to the extremity, as in grasses and palms (fig. 89), or from the midrib to the margin, as in Bananas (fig. 13, p. 33), and Indian-shot. The first kind of leaf occurs in dicotyledons or exogens, the second in monocotyledons or endogens. This constitutes another means of discrimination between these two great classes of plants, and is one which can be easily detected by the student of nature.

93. Sometimes the veins of leaves become hardened at their extremity, and project in the form of thorns, as seen in the holly (fig. 90), and the barberry. The same remark may be made in regard to them as has been applied to other thorns. They are produced by an arrestment in the development of the cellular part of the leaf, and a change in the structure of the veins. In the holly, we see that in certain circumstances it produces spineless leaves, in consequence of the cellular tissue filling up the spaces between the veins, even to the very edge. The poet has alluded to this in the following lines, but he has certainly given reins to his imagination in attempting to give a reason for the difference in the two kinds of holly leaves:—

"O reader! hast thou ever stood to see

The Holly-tree?

The eye that contemplates it well, perceives
Its glossy leaves,

Ordered by an Intelligence so wise

As might confound the Atheist's sophistries.

Fig. 90.-Holly, showing the spines of the leaves depending on nondevelopment of the cellular tissue at the margin, and the hardening of the extremities of the veins.

"Below a circling fence its leaves are seen
Wrinkled and keen;
No grazing cattle thro' their prickly round
Can reach to wound;

But as they grow where nothing is to fear,
Smooth and unarmed the pointless leaves appear."

94. The surface of leaves called stomata (fig. 91, st;

st

Fig. 91.

presents certain pores, and fig. 87, p. 116, s t) The cells surrounding these pores are so constructed that in dry weather they collapse, and close the opening; while in moist weather they have a crescentic margin, by which they open the orifice. They are connected with the passage of air and fluids to and from the leaf. In fig. 91 they are seen scattered over the surface at re

gular intervals. They are easily seen by putting a very thin piece of the skin of the leaf of a hyacinth or lily under the microscope. They vary much in their form and appearance in different plants.

Fig. 91.-Stomata st, or openings in the epidermis or skin of the leaf of Balsam. There are five of these represented in the figure, placed at regular intervals. They are connected especially with the exhalation of fluids.

95. In the form and size of leaves we may perceive many interesting adaptations. Thus the large fan-shaped leaves of palms are fitted for shade and shelter in the warm countries in which they grow; while the narrow leaves of pines and firs fit them for the alpine districts in which storms and blasts prevail. In leaf-buds, also, wonderful provision is made by the Creator for the preservation of their contents. They are a sort of winter-quarters, in which the young leaves and branches are nursed. With this view, they are covered with coarse external leaves, or with a coating of gummy or resinous matter. It is only when the genial warmth of spring calls them forth that they burst their cerements, and expand their delicate structures to the air. Our native plants protrude their leaves cautiously, and thus are seldom injured much from our variable springs; but exotics transplanted from temperate climes, where spring is continuous, and there are no nipping frosts to arrest growth, are often, as it were, deceived by a few days of warmth in our northern climate, put off their winter clothing too soon, and thus suffer severely for their temerity. The time of putting forth the leaves indicates the nature of the seasons, as well as the time of the falling of the leaves. On this subject Dr Fleming has entered, in his excellent work on the curves of temperature, which has appeared as one of the series of the

"Christian Athenæum," and it is needless to dwell it here.

upon

This conThe amount

96. The function of the leaves is to expose the juices of the plants to light and air, and thus to aid in forming the woody matter of the stem, and the various secretions. Unless the leaves are freely exposed to air and light, the wood is not properly formed. Hence the reason why the wood is deficient both as regards quantity and quality in trees grown in crowded plantations. The same observations apply to all the secretions of plants. Thus potatoes grown in the shade, by which the functions of their leaves are impeded, become watery, and produce little starch in their tubers. Leaves also give off a quantity of watery fluid. stitutes what is called exhalation. of fluid exhaled varies according to the structure of the leaves and the nature of the climate. When the texture of the leaf is hard and dry, as in some Australian plants, or the skin covering the leaf is thick and dense, and has few stomata, as in the American aloe and in the oleander, the exhalation is comparatively small. In this way certain succulent plants, as cactuses, are enabled to withstand the effects of dry and hot climates without being dried up by the great loss of fluid from exhalation. The thick covering of hairs, as well as the waxy coating on some leaves, seem to be connected with the amount of exhalation.