THE PREVENTION OF DROUGHT.

163

"During summer, a mean monthly difference of five and even six degrees is of considerable importance; and even supposing it to be reduced to half that amount by the time the sap has reached the leaves, it still lowers very perceptibly the temperature of the whole plant. This effect is increased by that evaporation from the leaves which results from the descent of the sap during the day; so that the hotter the air is, the greater are the resources which the plant finds within itself, and the earth in which it grows, for diminishing the effect of external heat."

The prevention of drought. Mr. John Sincham, in his work for the use of the engineer, thus enumerates the effects of pulverization: "It makes the drains efficient almost immediately and permanently. It enables the crop to be planted and harvested much earlier. It is the first and great step towards increasing the depth, and improving the condition of all light, bad soils. It allows the atmosphere to penetrate and circulate freely through the soil, and admits carbonic acid. gas and oxygen to the roots of plants, to act upon the deleterious ferruginous compounds and secretions to be found in most soils. It secures also to them benefit from every drop of rain that passes through the atmosphere."

CHAPTER VI.

FERTILIZATION.

SELECTIVE POWER OF PLANTS -FORM IN WHICH FOOD IS TAKEN UPEXCRETION CIRCULATION OF THE SAP- ORGANIC

SUBSTANCES:

OXYGEN, HYDROGEN, CARBON, NITROGEN — INORGANIC SUBSTANCES — MANURES ADAPTED TO DIFFERENT SOILS: TO CLAYS, TO PEATY SOILS, TO SANDY LANDS-PLANTS TO BE USED IN GREEN-MANURING APPLICATION OF MANURES.

PLANTS, like animals, show a preference in the selection of their food. They reject some substances, and elect others adapted to their nourishment.

Their food is supplied in two forms — liquid and gaseous. It has been stated by some writers that the former is absorbed by the roots, and the latter exclusively by the leaves. While this in the main is in accordance with observation, yet the roots do also absorb gaseous food, often in considerable quantities, especially in a well pulverized and thoroughly drained soil.

The cells through which the sap flows are, in their original and simplest form, cellulose, or parenchyma. Toward the root, the sap in these cells is thin; but at the leaves it becomes thickened by the decomposition of its water and the elimination

CIRCULATION OF THE SAP.

165

of its oxygen. Thus there is constantly going on between the cells a process like that which may be observed if a membraneous bag containing a thick fluid should be immersed in water, - the thick becomes thinner by the absorption from the latter, or endosmose, while the water derives a small quantity of the former. Hofmeister discovered that this was the cause which produced the circulation of the sap from the root to the leaf, and that the little white fibres, styled spongioles by old writers, which were emitted during each season of growth, were formed of cells not covered by the thick skin or epidermis of the older roots, and that they absorb the thin fluid of the soil, which contains pabulum in solution. Thus a regular grade of elaboration, or thickness of the sap, is established from the leaf to the roots. These absorbing cells, after having performed their work for one season, become covered with the epidermis, and cease to provide for the growth of the tree. This should be borne in mind by the cultivator when he applies his manure to the ground about his trees. The feeding roots are at a considerable distance from the trunk of the tree, and if applied near it, scarcely any benefit will

accrue.

Roots have been said to excrete substances absorbed, which were unnecessary or injurious. This was the belief of De Candolle, and others. It was formed from the fact that a plant will not succeed

166

SELECTIVE POWER OF PLANTS.

where one of the same species had been previously growing for any length of time, and that substances existed in that soil which were not to be found in other parts of the field. But this is by no means a sufficient proof, because we should expect that if the existing combinations were separated, others would be formed.

That the roots possess a general power of selection, we are led to believe from the fact that the ashes of various species differ so widely even when the plants are grown upon the same soil. It is probably confined to the combinations which the plant meets with in its indigenous soil, and does not apply to those artificial substances which are sometimes applied, nor to the tree when placed in an unnatural position. Thus, the application of a large quantity of iron'to the roots of a plum tree, resulted in a dark-colored exudation from its bark ; and the colors of flowers are often changed by the substances placed in the soil.

It will be well to examine more particularly the structure of those organs of the plant which perform the function of digestion. The upper surface of the leaves, and all green parts of other portions of the trec, exhale the oxygen contained in their sap, while the under surface of the leaves inhales carbonic acid and some other fertilizing gases from the atmosphere. The cells, on the upper surface, are therefore very close, to prevent excessive evaporation,

THE MULTIPLICATION OF CELLS.

167

while on the under surface they are placed loosely to admit gases freely. Thus this figure 1 represents

a cut made through the thickness of the leaf, exhibiting its edge highly magnified.

(a) denotes the epidermis;

(b) a close layer of parenchyma; (c) internal cells;

(d) the cells of the lower surface;

(e) stomates, or valves through which the gases

enter.

As the sap descends in the liber by gravitation, it causes the multiplication of cells between the bark and wood. Every cell contains within itself a nucleus, from which the sap flows toward the walls; this contains several nucleoli, or cytoblasts, which, when they have attained size, burst the nucleus, and each becomes like its parent. Consequently, if there are two nucleoli, each creates a current acting counter to the other, and at their point of contact a membrane is formed which divides the old cell. This is very rapid work. We can

1 Dr. Lindley's Introduction to Botany.