The Vegetable World: Being a History of Plants, with Their Botanical Descriptions and Peculiar Properties

In order to test this phenomena, let us fill a bell glass with water, to which has been previously added a considerable proportion of carbonic acid gas, and place in it a branch or an entire plant covered with leaves; expose the whole to the rays of the sun for some hours, as is represented in Fig. 134. The air, if analysed after the experiment, will be found to contain scarcely any carbonic acid, but it will contain a larger portion of oxygen than before the experiment. Ifa branch of a plant, with the roots fixed in soil, and consequently in its normal state of vegetation, is placed in a glass vessel, and by means of an air-pump a given quantity of air is caused to circulate round it, this air, which, before the experiment, contained from four to five ten-thousandth parts of carbonic acid, after the apparatus has been exposed to the influence of the sun’s rays for a certain time will not be found to contain more than from one to two. If, on the contrary, the experiment is made during the night, it will be found that the quantity of carbonic acid would be increased, and at the expiration of a certain time

would have risen to eight ten-thousandth parts. These experiments, in which there is an interchange of gas between the plant and the atmosphere, exhibit the double phenomena of absorption and exhalation in plants; in fact there is respiration. But the respiration of plants is not always the same, like that of animals, in which carbonic acid gas, water, and vapour is exhaled without cessation either by day or night. Plants possess two modes of respiration: one diurnal, in which the leaves absorb the carbonic acid of the air, decompose this gas, and extract the oxygen whilst the carbon remains in their tissues; the other nocturnal, and the

reverse, in which the plant absorbs the oxygen and extracts the

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carbonic acid; that is to say, they breathe in the same manner 4s animals do. The carbon which is used by plants during the day is indispensable to the perfect development of their organs and the consolidation of their tissues. By respiration plants live and grow. It is necessary to remark here that it is only the green partsof vegetables which respire in the manner described; that is to say, by absorbing carbonic acid and disengaging oxygen under

the influence of light. The parts not coloured green, such as the wall fruit, seeds, red and yellow leaves, &c., always respire in one and the same manner: whether exposed to light or left in darkness, they always absorb the oxygen and disengage carbonic acid. They respire in the same manner as animals. If we consider that the green parts of the plant are far more numerous than those which are otherwise coloured,—that the clear light nights of hot countries may rather be said to diminish than to interrupt their respiration,—that the season of long days in northern countries is that of the greatest vegetative activity—we shall be led to the conclusion that the great mass of plants live more in light than in darkness, and consequently that their diurnal respiration greatly preponderates over their nocturnal. e diurnal respiration of plants, which pours into the air considerable quantities of oxygen gas, happily compensates for the effects of animal respiration, which produces carbonic acid gas, injurious to the life of man. Plants purify the air injured by the respiration of men and animals. If animals transform the oxygen of the air into carbonic acid, plants take this carbonic acid back again by their diurnal respiration. They fix the carbon in the depth of their tissues, and return oxygen to the air, in reparation. Such is the admirable equilibrium which the Creator has established between animals and plants, such the beneficial communication which assures to the air its constant soundness, and maintains it in that state of purity which is indispensable to support the life of the living creatures which cover the globe.

We have now been speaking of the respiration of aerial plants. Water plants cannot respire by the same organic mechanism. In these the air circulating across the intercellular meatus of the leaves acts directly upon the pithy contents of the’ cells. The leaves of aquatic plants, which are destitute of epidermis, and are in general very slight, borrow air which the water holds in solution, m such a manner that, according to the ingenious remark of M. Brongniart, they respire in a manner analogous to that pre- | sented by fishes and other animals which breathe by gills.

Plants which live in perpetual darkness,and which consequently are always subject to nocturnal respiration, maintain certain modifications in: their exterior aspect. In this anomalous condition

they lose a great part of their carbon, which passes into the state of carbonic acid, and they exhale large quantities of water. The result of these two phenomena is a decided elongation of the plant, a greater softness in its tissues, and the absence of green colouring. The juices of plants kept constantly in the dark modify themselves in a sensible degree. Often acrid and bitter in their normal condition, they are rendered sweet and succulent under such modifications, and market gardeners turn these facts largely to their profit. They set Lettuce plants in artificial stations in order to make the hearts white, by binding them closely together and tying the leaves one against the other, to the exclusion of light. Sea Kale, which grows a wild, useless weed on the sea-shore, becomes @ delicate dish under the gardener’s care, aided by this principle of blanching. '

CIRCULATION.

The manner in which nourishing juices circulate in the interior of plants has long been the subject of discussion amongst botanists, and science is still far from being agreed on this important point of vegetable physiology. Limiting ourselves, however, to the consideration of dicotyledonous vegetation, such as our indigenous forest trees, we may enunciate the following simple facts on which all botanists are agreed. Let us follow the juices of a dicotyledonous plant from the moment of their absorption by the root fibres. Let us see the course these liquids follow as they rise in the interior of the plant, and that which they take in order to descend again after having passed through the pervious tissues of the leaves, exposed to the chemical influence of the air; in other words, let us follow the progress of the sap both in its ascending and descending course. From the moment when the water which impregnates the earth has penetrated into the roots of a plant, and mingled its sap with the juices which are contained the cells of the vegetable, it constitutes what botanists call the sap, a complex fluid, which, at certain periods in the life of a plant, circulates constantly through its tubes. What is the force which causes the water to penetrate into the roots, impels the sap into the

stem, and finally to its last ramification, namely, the leaves? What is the route which the sap follows in ascending? Does it traverse the pith, the bark, or the wood? or, finally, does it permeate through all three at the same time? When a tree is cut down in spring it is easily seen that the sap flowing in it is then in the wood. If a plant is made to absorb coloured liquid, or if the branches are plunged into the same liquid, it is easily seen that it does not rise first either in the bark or pith. It is the wood or ligneous body through which it manifestly takes its passage. This passage is effectuated through all the ligneous elements, cells, fibres, and vessels. The anatomical construction of these vessels, their large number, their strength in the prostrate filiform and slender stems, which often attain a very considerable length, and which require to be traversed by a large quantity of sap in order to supply what is necessary for evaporation by the leaves,—all these general facts leave no doubt as to the part which the wood vessels play in the circulation of the sap. There is besides nothing easier than to ascertain directly the presence of sap in the interior of the wood. Such, then, we may conclude, is the true path followed by the ascending sap. Dr. Hales, an English physiologist, to whom science is indebted for numerous experiments throwing light upon the movement of the nourishing juices in plants, was anxious to discover the force with which the sap rose in the stem. In order to ascertain this, he fastened a bent tube on the top of an ascending branch of a vine-stem in the spring; having carefully fixed the tube upon the transverse section of the vine-stock, the lower bend being filled with mercury. The flowing sap accumulated in the interior branches began little by little to move the mercury, till it rose to about forty inches in the tube (Fig. 136). The flow of sap then

was sufficiently powerful to move a column of mercury, in addition to the weight of the atmosphere, this height. Hales calculated from this that the force which impels the sap in the vine is five times as great as that which impels the blood through the large arteries of the horse. Having reached the leaves, the sap comes in contact with the air by the innumerable openings or stomates which communicate with the air-cells and hollow meatus in the substance of the parenchyma. The respiration of the plant, that is to say, the chemical action which the air exercises upon the liquids which supply the leaves, together with the exhalation of vapour which proceeds from the same organs, modifies the ascending sap in the same manner that air modifies the veinous blood in the sanguinary vessels and lungs of animals, changing it into arterial blood. Thus it is with the leaves in the phenomena of exhalation and respiration, of which they are the seat. In ascending, the sap changes its nature, and becomes transformed into a nourishing fluid. We must now inquire what is the path which this new descending sap takes after it has undergone this important modification. Everything tends to the belief that it circulates in the bark. Let us see what are the facts which justify this opinion. If a stem or branch is strongly bound in such a manner as to compress the bark, a cushion or excrescence is formed above the ligature, which continues to increase, and appears to proceed from the stoppage of the circulation, showing that the nutritious fluids come from above, for the parts beneath the ligature show 20 increase. The same phenomenon is produced upon the trunk of the tree when annular or spiral incisions are made all round it. Again, the trunks of trees round which creeping or twining plants wind themselves, demonstrate the same physiological fact in a very peculiar manner. Above the natural ligature produced by the pressure of the parasitic plant, a natural cushion is formed, and @ swelling produced by stopping the sap in its supposed descent through the bark. _ In speaking of the structure of roots, we have enumerated th causes under the influences of which the sapascends; the causes which determine its descent are, we must acknowledge, imperfectly know? to us. It appears most probable that it is through the deep layers of the bark, especially through the net-like fibres of the “ier, the

admirable structure of which we have before mentioned, that this sap takes its way. These fibres are very rich in a mucilaginous and albuminous matter. Some physiologists consider that there are additional vessels as principal and essential reservoirs for the elaborated sap, but it must be confessed that the subject is very obscure. The circulation of sap is in its most active state in spring time, when the plant is gorged with nutritive matter preserved in deposit during the winter. It is then full of liquid, and in some plants the juices flow from the slightest incision. In spring, according to the poetical expression consecrated by use, the vine and other plants bleed; but when the leaves are fully developed, the active evaporation which takes place on their surface impels the liquid to the extremities of the vegetable, whence it exhales in vapour; they will no longer bleed when wounded.