aquatic plants in their attempts to gain a footing on dry land, favoured by the method for preventing dessication described above, this idea was replaced by a second, which proved so successful that it has never been superseded, and is at the present day the universal method adopted by all forms of terrestrial vegetation. This successful effort depended on division of labour, the one idea more than all others that has enabled life to reach its present phase of development, and consisted in the external cells of those portions of plants growing in the air becoming converted into a waterproof epidermis or skin, the various structural peculiarities of which will be explained at a later stage. The protective organs of plants may be conveniently referred to under two headings.

(a) Protection against climate. Under this heading may be mentioned hairs, movements (as the closing up of leaves and flowers), colour, etc.

(b) Protection against living enemies. Stings, spines, bitter tastes, colours, &c. It is important to remember that the above divisions are not sharply marked in nature; for example, colour may be protective against both climate and living enemies; the same is true of various scents and secretions. Flowers may close during a rainy day to prevent their pollen being washed away, or the closing may take place at certain periods to prevent the entrance of insects not adapted to effect fertilization of the young seeds. Many contrivances evolved obviously for protective purposes by plants are rendered more or less useless by the cunning of living enemies; for example, the common wayside weed,. Glechoma hederacea, or Ground Ivy, has a corolla prolonged behind into a tube which secretes honey for the purpose of attracting insects in connection with cross fertilization.

Owing to the length of the corolla-tube, which is furnished inside with a ring of hairs pointing towards the centre, only long "tongued" insects, as some kinds of bees, can reach the honey; in other words, the arrangements are such that only those insects that are adapted for effecting fertilization by removing the pollen from one flower and depositing it on the stigma of another can obtain the honey by legitimate means. But certain insects that could not effect the fertilization of the plant are conscious of the existence of honey, and not being able to reach it by the usual channel, bite through the corolla-tube from the outside at the point where the honey is secreted, and thus obtain a meal by surreptitious means. On the other hand, instances of protection afforded by members of the animal kingdom to certain plants has gradually evolved into mutualism, or that condition of things where two living organisms, originally independent of each other in every way, become so modified as to live in each other's society with mutual advantage.

A striking illustration of mutualism is described by Belt (The Naturalist in Nicaragua) as follows. "A species of Acacia, belonging to the section Gummifera, with bi-pinnate leaves, grows to a height of fifteen to twenty feet. The branches and trunk are covered with strong curved spines, set in pairs, from which it receives the name of 'Bull's-horn Thorn,' they having a very strong resemblance to the horns of that quadruped. These thorns are hollow, and are tenanted by ants, that make a small hole for their entrance and exit near one end of the thorn, and also burrow through the partition that separates the two horns, so that the one entrance serves for both. Here they rear their young, and in the wet season every one of the thorns is tenanted, and hundreds of ants are to be seen running about, especially

over the young leaves. If one of these be touched, or a branch shaken, the little ants swarm out from the hollow thorns and attack the aggressor with jaws and sting. They sting severely, raising a little white lump that does not disappear in less than twenty-four hours. These ants form a most efficient standing army for the plant, which prevents not only mammalia from browsing on the leaves, but delivers it from the attacks of a much more dangerous enemy the leaf-cutting ants. For these services the ants are not only housed by the plant, but are provided with a bountiful supply of food; and to secure their attendance at the right time and place, the food is so arranged and distributed as to effect that object with wonderful perfection. The leaves are bi-pinnate. At the base of each pair of leaflets, on the mid-rib, is a crater-formed gland which, when the leaves are young, secretes a honey-like liquid. Of this the ants are very fond, and they are constantly running about from one gland to another, to sip up the honey as it is secreted. But this is not all; there is a still more wonderful provision of more solid food. At the end of each of the small divisions of the compound leaflet there is, when the leaf first unfolds, a little yellow fruit-like body united by a point at its base to the end of the pinnacle. Examined through a microscope, this little appendage looks like a golden pear. When the leaf first unfolds the little pears are not quite ripe, and the ants are continually going from one to another, examining them. When an ant finds one sufficiently advanced it bites the small point of attachment; then, bending down the fruit-like body, it breaks it off and bears it away in triumph to the nest. All the fruitlike bodies do not ripen at once, but successively, so that the ants are kept about the young leaf for some time after

it unfolds. Thus the young leaf is always guarded by the ants, and no caterpillar or larger animal could attempt to injure them without being attacked by the little warriors. The fruit-like bodies are about one-twelfth of an inch long, and are about one-third the size of the ants, so that the ant bearing away one is as heavily laden as a man bearing a bunch of plantains. I think these facts show that the ants are really kept by the Acacia as a standing army to protect its leaves from the attacks of herbivorous mammals and insects." The above detailed account will give an idea as to the amount of specialization entered into by some plants in their endeavours to insure protection. Several other plants belonging to widely separated families have arranged on mutual terms with ants for protective purposes, suggesting that the idea has originated independently in different groups of plants.

The substance of plants is not homogeneous, but consists of minute portions, for the most part invisible to the unaided eye. These minute fundamental parts are called cells, and it is important to remember that every variety of form and texture, as also the various colours, secretions, scents, etc., depend entirely on division of labour and differentiation in cells. The term cell implies a closed vesicle, and consequently clearly expressed the idea entertained by the early botanists as to the structure of this organ, but at the present day many fundamental parts are known that are not covered by the older definition; nevertheless the old name is retained.

A typical cell presents the following structure: (a) an external protective membrane called the cell-wall, which is composed of a substance formed by the protoplasm, and having, at least when young, a constant chemical composition. This substance is known by the name of cellulose.

(b) Enclosed within the cell-wall is the protoplasm, which in the young condition almost entirely fills the cavity. (c) In the protoplasm of all young cells of the higher plants, and in most of the simpler forms also, there lies imbedded a more or less spherical body called the nucleus.

When quite young all cells agree in general structure and appearance; but owing to the division of labour already alluded to, the mature cells differ very much from each other both in structure and function.

In young cells the wall is very thin, but owing to the continued secretion by the protoplasm of the substance of which the cell-wall is formed, the latter continues to increase in area and thickness. The growth of the cell-wall is entirely dependent on the contact of the contained protoplasm with its inner surface, consequently cells from which the protoplasm has disappeared cease to grow, and as all vital functions emanate from protoplasm, such cells can only perform mechanical functions, as giving strength or rigidity. This is clearly seen in the case of hollow trunks, the heartwood or central portion is the oldest, and in old trees the protoplasm has almost entirely disappeared from the thickwalled cells of this part, and their decay and total disappearance does not at once cause the death of the tree, the necessary communication between the root and the leaves being carried on by the young wood lying just below the bark.

In young cells the thin wall consists mostly of cellulose, a substance having the same chemical composition as starch (CHO), in addition there is always a considerable quantity of water, and a varying but always very small proportion of mineral matter which remains as ash when the cell-wall is burnt.