bed. This is for the purpose of giving them a stimulus at exactly that time when they are most ready to receive it. Had they been forced at first in bottom heat, the stimulus would have been applied to cuttings whose excitability had not been renovated (113.), and the consequence would have been a developement of the powers of growth so languid, that they probably would not have survived the coming winter: but, the stimulus being withheld till the cuttings are quite ready for growth, it tells with the utmost possible effect.

In addition to these comments upon an excellent mode of striking cuttings of many kinds, it is necessary to add some observations upon the object of additional precautions often taken by gardeners.

Cuttings are covered by bell glasses, whose edge is pressed into the earth. This is for the purpose of preserving a uniform degree of humidity in the atmosphere breathed by the cuttings. It is generally necessary to leave one or more leaves upon a cutting, in order to generate organisable matter, and to assist in the formation of roots; but this is a very delicate operation, for, if the leaf is allowed to suffer by excessive perspiration, the cuttings must necessarily perish (75.). To maintain a steady saturated atmosphere around a cutting stops this danger, and hence the use of a bell glass. A double glass has even been recommended (fig. 23.); but, if this precaution is of any value, it must be, not

J

because it maintains an even temperature, which is injurious rather than useful, but because it prevents condensation upon the inner bell glass, and the

23 consequent abstraction of atmospheric moisture, and probably acts at the same time as a kind of shade.

Notwithstanding the precaution of covering cuttings with a bell glass, shade is also necessary, as a further security against perspiration; for light acts as a specific stimulus (71.), whose effects are very difficult to counteract. It must, however, be employed with great caution; for, if there is not light enough, the leaves attached to the cuttings cannot form that organisable matter out of which roots are produced.

All gardeners know that the root end of a cutting should be close below a leaf-bud; this is to facilitate the emission of roots by the buds, which emission must necessarily take place with greater or less difficulty in proportion as their exit is facilitated or impeded by the pressure of bark on them.

No further precautions are taken with cuttings, nor does it at first sight appear possible to suggest any nevertheless, the enormous constitutional difference among plants is such, that, while numerous species will strike without any difficulty under almost any circumstances, with the wood ripe or

half-ripe, just formed or aged, there are many others which no art has yet succeeded in converting into plants; and it is by no means uncommon to find that, out of a potful of cuttings of the same species, apparently all alike and subjected to exactly the same treatment, one will grow and the remainder fail. It is, therefore, one of the most probable of all things, that the principles of striking cuttings are still very imperfectly understood, and that this is one of the points of horticulture in which there is the greatest room for improvement.

It may be worthy of enquiry whether bell glasses of different colours will not produce different effects upon cuttings, in consequence of their different power of transmitting light. It has been shown by Dr. Daubeny, in a very interesting paper in the Philosophical Transactions for 1836, page 149., that glass of different colours exercises very different effects upon the plants exposed to the rays of solar light passing through it; that both the exhalation and absorption of moisture by plants, so far as they depend upon the influence of light, are affected in the greatest degree by the most luminous rays, and that all the functions of the vegetable economy, which are owing to the presence of this agent, follow in that respect the same law. In these experiments it was ascertained that the glass employed admitted the passage of the rays of light in the following proportions:

M. Decaisne found, during some experiments to ascertain the effect of light in causing the production of colouring matter in the Madder plant, that when the lower parts of a plant were enclosed in cases glazed at the side with transparent green, red, or yellow glass, the leaves and stem of the part surrounded by red glass became pallid, and exhibited signs of suffering in a greater degree than under the other colours, but all were affected more or less." (Recherches sur la Garance, p. 23.)

We however require very different experiments from any yet instituted, before we can proceed to draw practical conclusions as to the relative effects upon plants of glass of different colours. We do not know what the effect is of the calorific and chemical rays, and therefore we cannot say what may be the advantage or disadvantage of their action upon plants. As, however, the object of the cultivator is to protect his cuttings from too much light, and at the same time to give them enough to enable them to perform their digestive functions steadily, there can be little doubt that transparent glass is inferior to that of another colour.

The nature of these experiments has been misapprehended in the translation, by Mr. Francis, of Meyen's Report on Vegetable Physiology for 1837, p. 51.

CHAP. XI.

OF PROPAGATION BY LAYERS AND SUCKERS.

WITH regard to layers, there is but little which it is necessary to say regarding them, if what has been stated respecting eyes, leaves, and cuttings, has been rightly understood and well considered. A layer is a branch bent into the earth, and half cut through at the bend, the free portion of the wound being called "a tongue." It is, in fact, a cutting only partially separated from its parent.

The object of the gardener is to induce the layer to emit roots into the earth at the tongue. With this view he twists the shoot half round, so as to injure the wood-vessels; he heads it back so that only a bud or two appears above ground; and, when much nicety is requisite, he places a handful of silver sand round the tongued part; then pressing the earth down with his foot, so as to secure the layer, he leaves it without further care. The intention of both tongueing and twisting is to prevent the return of sap from the layer into the main stem, while a small quantity is allowed to rise out of the latter into the former; the effect of this being to compel the returning sap to organise itself externally as roots, instead of passing downwards below