lators kept close the atmosphere in the house may be kept at the proper temperature, and it will assist vegetation instead of destroying it. It should be remembered that in every glasshouse there is always top ventilation through the laps of the glass, and, generally speaking, I think a one-foot swing ventilator at each end of the house would meet the case, and not give the opportunities of abuse which the present system of top ventilation does. I am sure that many hybrid Orchids, and also many rare species, have departed from this cause alone.

Once well established, however, the home-raised Orchids have a decided advantage over the imported plants, in that they are much less liable to be affected by our long spells of dull weather and other climatic peculiarities.

THE

ADVANTAGES

ΤΟ

GARDENERS OF SOME

KNOWLEDGE OF VEGETABLE PHYSIOLOGY.

By the Rev. Prof. G. HENSLOW, M.A., F.R.H.S., V.M.H. [Substance of a Lecture delivered at the Society's Gardens, Chiswick, June 15, 1898.]

INTRODUCTION:

WHAT IS VEGETABLE PHYSIOLOGY ?—It is

the study of the natural history of living plants.

To secure health in a human being a doctor must understand the structure, functions, and requirements of every organ of the body; e.g. the lungs are so made as to absorb oxygen from the air for breathing, by which life is kept up; and one might make a somewhat analogous statement of a leaf-that it is so made as to absorb carbonic acid gas, by which plant structure is built up. Similarly, with regard to the digestive organs, a doctor must know the nature of all the organs of secretion, the relative values of various foods and how they can best secure the growth and development of the body as well as restore its waste. Similarly, to understand how a plant may grow well and thrive it is necessary to know the structure of its organs or members, what they do and what they require.

Some persons fancy that they can draw a sharp line between practice and science, or between cultivation and physiology. Such, however, is altogether a mistake; for all practical

gardeners are more or less scientific physiologists, though, perhaps, without knowing it. For, it is just because florists and horticulturists do succeed so wonderfully well in growing plants that they have discovered for themselves what their plants require; without, it may be, exactly knowing the why and the wherefore in every case. Their profession might, indeed, be called "practical vegetable physiology.”

What science can add to the gardener's knowledge and experience is, on the one hand, a full description of the minute anatomy of plants, discovered after long and careful microscopical examination of them; and secondly, the several functions of the different tissues, and thence the functions of all the plant organs which are built up of those tissues.

If, then, a gardener understands this, it is for him so to place his plants, and so to feed them, as to secure to the fullest possible amount the complete activity of each and all the plant organs.

It must always be remembered that no absolutely perfect conditions can be secured; but the cultivator can always endeavour to obtain approximately the best; and these the grower can often find out where a scientist has no opportunity of telling him. Thus, e.g., all green plants require light; but the amount that each requires as best for it is not the same for all. This the Aucuba japonica illustrates very well; for the leaves on the surface of a bush, and most exposed to light, are much more spotted with yellow, which increases with a prolonged intensity, than are the leaves more deeply situated within the bush. These are always of a darker green and have fewer spots. Conifers, too, often show a marked susceptibility to too strong a light, when the colour of the foliage turns to a yellow green.

The adaptations of plants to their surroundings should be studied much more than has been the case. Thus, when collectors introduce new plants from foreign countries, they should always record accurately in their note-books the conditions under which the plants were growing in their native homes; not only the nature of the soil, but the amount of light or exposure to the sun, the amount of moisture or its absence, &c.; because the natural and healthiest conditions of plants in the wild state are solely due to their having become thoroughly adapted to those conditions; and the nearer the cultivator can

imitate them, it is obvious, as a general rule, the more healthily will they grow. Thanks, however, to this very power of self. adaptation, they can accommodate themselves within limits to their new environments in a hothouse, greenhouse, or open border in England. Still, the more that is known about their habits in nature, so much the better for the cultivator. Thus, it is said that when Aucuba japonica was first introduced from Japan, it was grown under glass; but a stray plant having been thrown away was discovered to have lived through a severe winter, so that it has ever since been grown in the open. A gardener thus finds by experience, often after many mistakeswhich cost money how to grow foreign plants; but his knowledge may have been acquired laboriously; and he may even then not know why it is best to do this or that for his plants. It is here that a knowledge of physiology can sometimes step in and tell him; for it is the province of science to investigate into causes. Nature, however, and especially the department of "life," is so obscure that no scientist has ever probed all her secrets to the bottom, and probably never will be able to do so; until she tells us how it is that life-forces can issue out of food. However, no one can cultivate a plant at all without knowing something of its physiological requirements to enable it to live and thrive; and just so far as the practical man succeeds, so far is he acquainted with the main facts of physiology. The question is, therefore, Does he know enough?

of

ILLUSTRATIONS.-It will not be amiss to review the organs a plant in order to consider briefly their functions in a general sort of way. It will then be seen that there is really no great mystery in physiology after all. In order to illustrate the preceding remarks let us commence with a few physiological observations on germination.

Germination.-Seeds must be moist throughout. There must be a proper temperature; and a free circulation of air. The first and second are necessary in order that the various chemical changes may take place within the seed; the third is required for respiration. This shows the importance of avoiding too great a depth or too wet a soil. The radicle, on protruding, points earthwards. This is due to gravity. If the radicle be placed in a horizontal position, gravity acts on the tip; but the influence is conducted to a certain distance behind it, where the

curvature downwards at once commences.

If the tip be broken off, gravity ceases to act upon the root, and fresh roots have to be made, and consequently dangerous delays may occur in transplanting seedlings.

Roots and their Functions.-The absorbing surface is confined to a certain but short distance immediately behind the apex, and consists of the delicate epidermal surface with its root-hairs if present. This is easily proved by a simple experiment of placing a Radish plant with its tip only in water; the leaves, &c., will keep quite fresh. If, however, it be so bent, e.g., in a tumbler of water, as to submerge the greater part of the root, leaving the tip outside, the foliage will soon wither. All these facts demonstrate the vital importance of keeping as many as possible of the extreme tips of the roots intact, when transplanting herbs or shrubs. Another physiological fact about roots is their occasional production of buds. Roots and stems are fundamentally the same thing; but, as a rule, stems produce buds, because they grow in air and light. Roots, as a rule, do not; but if they be superficial, as in many trees, they frequently assume this function. Many plants have acquired the habit of doing it habitually. Elm trees by a road side in time make a perfect hedge over a superficial root. Raspberry roots have the permanent habit of throwing up buds. So that if buds are required from roots, they can be stimulated to bear them, as gardeners have discovered it to be possible in Peaches, Maclura, Paulownia, &c.

Leaves and their Functions.-The two chief functions specially characteristic of leaves and other green parts are transpiration and assimilation; for leaves are, so to say, the breathing, digesting, and perspiring organs of plants. To carry these functions on satisfactorily, cleanliness of the surface is desirable, for they are executed by means of minute pores, or stomates, in the epidermis. These two functions depend upon light; but before assimilation can be carried on at all, it is necessary for the leaf to be green. This can be effected only by light in ordinary plants. Light is compounded of many rays-such as the invisible, but sensible, heat rays, the ultra-violet invisible rays, and the intermediate coloured rays visible to our eyes. These range from red through yellow, green, and blue to violet, with intermediate mixtures. It is found that light of any colour will

cause the green chlorophyll to appear, but unequally so. The use of the chlorophyll granules is to decompose carbonic acid gas absorbed from the atmosphere, of which it forms, on the average, about one twenty-fifth part per cent. From this the carbon is retained for making tissues, while the oxygen is liberated. It is found that only certain parts of the solar spectrum are specially concerned in this process, though probably others assist. Such are some of the brightest parts of the spectrum, as well as some in the blue portion; but any specially coloured glass taken alone is injurious to plant life. The ill effects can be seen by growing Lettuces or other plants in frames under coloured glasses, when it will be found that the red and yellow glass tend to elongate the stem, as takes place in the dark. Green glass proves to be the very worst colour of all. As examples of a mistaken view, Mr. Decimus Burton constructed the Palm-house of Kew with a glass slightly tinted green, according to Dr. Daubeny's advice. This was with the idea of reducing the glare. Fortunately the tint is so slight as to do no harm if it does no good. It proved otherwise with the Fern-houses at Kew. The green glass of a deeper tint with which the Ferns there were covered, proved to be so injurious that it has been replaced with ordinary transparent glass. The fact is that plants have been attuned by Nature to the whole body of light, and it is only a question of its being either too intense or insufficient. If, therefore, the light has to be subdued, it must be done by some white material that reduces the amount by reflection, or otherwise, without decomposing the light itself. Such are the suggested practical results from physiological experiments.

Transpiration, though carried on by the colourless living protoplasm everywhere throughout the plant, is intensified by green chlorophyll, and like assimilation is largely dependent on certain rays. It must be distinguished from evaporation which results from heat. All dead and moist substances will evaporate, but only living plants transpire. As this function is most active when there is much foliage, it is easy to see how undesirable it is to transplant herbaceous plants when in full vigour, as the check to absorption by the roots can only be overcome by supplying a superabundance of water until the plant has established itself.

Flowers.-Coming to the reproductive organs, the process of

F