The Sap of Trees.

Thames Star, Volume XXII, Issue 6749, 5 December 1890, Page 4

The Sap of Trees.

A keen observer and ingenious experimentalist, writes the Globe, has been writing a book on "Sap: Does it Eise from the Boots ?" a question which he proceeds to answer with a decided negative. To those who merely " run and read,'* the whole question and answer may appear as a very slight matter indeed. In reality, however, the right answer to the question is of greai moment to us all. In the first place, a negative reply simply means the ruthless upsetting of all our preconceived ideas, the reversal of what has been taught in schools for ages. \ ''■ *

It is trad that, if we examine into the theory of plant growth as set down by botanical and biological authorities, wo find that they disagree with each other to an astonishing degree. They all, however, join in declaring that trees and plants derive sap from their roots, and breathe in gases by their leaves. How the sap riies, whether by capillary attraction, endosmoae, root-pressure, (faction, or evaporation, or a combination of all (decribed by Professor Huxley as polling, pushing, and pumping), tho greatest bio logists, including Herbert Spencer, Sachs, Huxley, Darwiu, and others, have by no means been able to decide or prove. They all nevertheless endorse the theory of rising sap, and agree moreover that it rises in spring and descends in summer. Wow, if all this theorising is proved to be wrong, we shall not only have to alter our school teaching, but largely modify oar agricultural practice. Clearly if the rootß suck up, absorb, or otherwise collect moisture, and the orgaDio or inorganic constituents of sap from the soil, then the soil will quickly fcvcotto , exb*u»tod aod require oooitaot

feeding, while the leafy parts of she plants mnet be cat and pruned down. This is, the present pracu'ce. Arid it is certainly worthy of remark that our best cultivated—according to the theories of fhe day—orchards and gardens are those that sniffer most readily Jfrom blight Jand disease generally. Oar new botanical revolutionist, Mr J. A. Reeves, with his book on " Sap," tries to prove that we are altogether on the wrong road ; that sap does not ascend, bat descends; and that gas does not descend, but asceods in all trees and plants. To the unprejudiced physicist and mechanician this theory of Mr Reeves seems by far the.most credible arid intelligible. It is, for one thing, strictly in accord with the proved and universally acknowledged laws of gravitation. Weall know that water naturally falls, arid can "only be made to rise under great pressure ; absorption and capillary attrac tion can only raise it some thirty feet in wet wood, practically not at all in dry wood. On the other hand, gases naturally rise, and to reverse the process great force, exceptionally applied, must be brought into play.

We are told off-hand that sap descends in autumn and rises in spriDg. Seeing the impossibility of getting liquids ab sorbed by dry wood, how can we account for tho alieged phenomena of rising sap in the dry trees in spring P In this connection it is remarkable that trees out down early in the year before they are in leaf will often sprout out some months after they have been severed from the roots and stacked away, Nobody has yet observed the rising of sap. Mr .Reeves has actually witnessed its descent. To do this, our experimentalist tested his theory by removing a ring of bark from a growing tree. Soon the sap was seen to accumulate on tho upper edge of the bark, while the lower remained dry; the sap gradually descended and hardened in the air. A branch cut close to the tree bleeds profusely in the upper part.

Testing theories; is always the safest plan. We fiud roots the driest parts of plants, the leaves the wettest. If we carefully exclude the leaves from moisture, the plants die, however much water may be applied to the roots. Tho roots are particularly dense in texturo; the loaves offer a large surface, covered with innumerable little hairs for the retention of moisture, and mou'h'(or stwiata) for its absorption. A great variety of trees and plants flourish in the most arid, sandy, and stony soil If we examine sap, we find that in the leaves and upper twigs and branches itisalmost perfectly limpid; it grows dense as it reaches the roots. This goes to show that the water, the principal constituent of sap, is taken in by the leaves and sent down to the roots; it grows thick as it descends by being exposed to the chemical influence of the rising gases. But it is not only water that is taken in by the leaves. The mineral constituents of the tree-blood also seem to be derived .from the air and not from the'soil. /Independent experiments on a wide icale have clearly deraonirated that our atmosphere is heavily charged with organic matter, dead and living, in organic atoms —the beams which we see in the sunlight rays of a darkened room. At all events the air contains abundance of mineral and other solid food for plants ; and when we fiod the leaves containing more .mineral matter and greater color than the roots, it seems hard to believe that our teachers have been over accurate in instilling into us the theory of rising sap.

Now, as to the gases. We must acknowledge that gases are plentiful enough in the air, but it is beyond dispute that those found necessary for the life and growth of plants are most richly elaborated in the soil. Plants with very deep roots are rarely fruitful. We know that soil dug up from great depths is practically dead, and will not afford a habitation for plants. Earth for successful plant growing must be impregnated with organic matter capable of decay, and consequently of evolving gas. This is an important point from the economic as well as the biological point of view. Fruit trees are found to prosper best and give the largest crops when their roots are near the surface of the soil. Indeed, some of the grandest orchards are partially paved at a depth of a few feet from the surface with large stones or hard-pressed debris, which forces the fruit trees to spread their roots near the surface of the soil, where the decomposition of natural vege table refuse and artificially applied manures are in the most active state of decomposition, and cons*yuenlly in soil richly impregnated with gases, Soots, while ill-suited for absorbing water, can easily tako ap gases. We find the roots comparatively free from coloring matter, and dry,, the Hap being thick. The gases io rising act chrinically upon the sap, the water and its organic and inorganic constituents, and finally escape through the leaves, and even the bark. That a very large amount of gas is given off by leaves, flowers, &c ,is an undeniable fact. We crush a leaf or twig between our fingers and obtain a more or less pleasant odour, and that this is due to gases can be easily proved, for many aromatic plants will Boon charge the atmosphere of a cloned room uncomfortably, sometimes even danger* ouslj, with compounds of oxygen, hydro* gen, and nitrogen , some of which are probably alkaloidal in character. Indeed, to such a large extent is this the case with certain plants that the natural escape of gas from tho rue plant on hot, still nights can be actually set on fire.

All this, it may be said, is very interesting ; but what practical lesson does it teach us? The groat practical lestoa is this—the ltaf is the most important part of the tree. If the plant procures its liquid and solid material for life and growth from the leaf, then clearly it is a mistake to be contioually lopping and pruning; for, by following this practice, we reduce the assimilating powers of the plants. This seems to be proved by the fact that heavily pruned trees are con< tinually throwing out suckers and shoots— in reality, endeavoring to extend their foliatfe in order, to compensate for tbeir mutilation. Cultivated trees that are pruned aud deprived of their lifegiving foliuge decrease, in strength, and fall easier victims to disease ■than' the heavily foliated trees in their natural mute. If, therefore we are to accept tho descending sap theory, we (shall huve to spare tho upper branches and cherish the leaves. Again if the sap, the water with its organic and inorganic matter, comes from the air, and the gases from the soil, then the recognition of such facts would have an undoubted in* flaence opon our present methods of agriculture, depending upoD arbitrary theories of soil exhaustion end rotation of crops. Agriculture will be as much a science as ever, but a more remunerative one. and our economics of the soil will heve to undergo pome modification.