another certain dissolved material requisite for its growth. The intercellular spaces are also subservient to the same purpose. The cells pour out their juices into an intercellular space, and they are then conveyed away by the open channels lying between

the cells. A gland [Fig. 6, B, d, e, g, h] differs from these latex vessels in being a part of no lactiferous system, they are local formations, which, when they are upon the surface layers, often discharge their secretions outwardly. The sap-conducting passages are intercellular spaces, usually arising from the separation of adjacent cells; both sap and resin passages lie generally in straight lines, or fol

low the fibro-vascular bundles. They resemble the lactiferous vessels in that they form continuous systems running through the whole plant. When they occur in the parenchyma, or fundamental cell tissue, they are generally distributed at nearly equal distances, forming a circle in the transverse section of the stem. [See Fig. 8.]

The whole epidermal system is covered and protected by a continuous cuticle which also lines the vertical walls of the stomata. This cuticle, in its turn, is further protected by a delicate coating of wax, the "bloom" so familiar to every one. Sometimes it forms a continuous coating; at others it stands as a multitude of upright or curved rods; again it appears in the form of scales, or of a thick incrustation, giving evidence of laminations like the cell walls, or of layers of granules, or even as erect, doubly refractive four-sided prisms.

We have glanced briefly at cell modification and development, and considered some of its physical peculiarities; but we have not even looked into its life-history, or the functions performed by it, for the organism of which it is a part. The entire life. of the plant depends on the action of light upon the cells, which contain chlorophyll. New organic compounds are formed only in these cells, and only under the influence of light. After a certain amount of assimilated material is stored up under the influence of the sun's rays, vegetative processes may, it is true, go on at their expense independent of the sunlight. This is especially true of seeds and tubers, which possess large stores of assimilated material

But in the darkness the plant hybernates, rather than lives in the full sense of the word.

The whole animal kingdom may be said to depend upon the vegetable, in two senses. The atmospheric equilibrium is maintained by their mutual action. Animals consuming oxygen and yielding up carbonic acid (carbon anhydride), and plants consuming carbon and liberating oxygen; in this sense the two kingdoms are mutually dependent, though not equally so; but in another sense animal life is entirely dependent upon vegetable. In the tiny cellulose sac, by the vegetable protoplasm is wrought the very alchemy of life. There alone the inorganic elements of earth, and air, and water are transformed into the only material capable of sustaining animal existence.

Late investigations into the processes of vegetable life go to show that plants, like animals, inhale and consume oxygen, and exhale carbon anhydride; but this process, which is continuous and gentle, is masked by a much more vigorous action, which takes place within the chlorophyll particles under the controlling power of light. While the regular breathing of the plant goes on, consuming and liberating the same elements as the respiration

of animals consumes and liberates, the chlorophyll particles perform a work somewhat akin to assimilation. The carbon anhydride of the air is seized upon by them, the elements are separated, the carbon is consumed, and the oxygen associated with it is liberated. The quantity of oxygen thus freed, and of carbonic acid thus disappearing, so greatly exceeds any effects which during the day the reverse process of breathing could produce, that the gentle exhalation has been entirely overlooked. The store of oxygen has been found to increase steadily in air removed from all influences but those of vegetation. The chlorophyll particles also store up the formed material: the formation of starch, oil, and sugar seems to be a function of chlorophyll exposed to light, and their absorption a function of chlorophyll in the dark.

Nothing is more familiarly known than that sunlight after passing through a three-sided prism is spread out into an almost infinite number of rays, whose colors fade gradually into one another. These seven colors, as for convenience they are called, do not constitute the whole solar spectrum; for beyond. the red end there is an invisible heat spectrum, and beyond the violet an invisible actinic spectrum.