FORMATION OF CELLS FROM GUMMY FLUID.

211

these, a delicate membrane is seen to project, just as a watch-glass projects from the face of a watch; and this membrane gradually extends much beyond the original disk, so as to form a kind of bag, in one wall of which that body is included. Still, the membrane is of so delicate a consistence, that it is easily dissolved away by shaking the vessel in which the process is being observed; and it is not until some time afterwards that it acquires any considerable firmness. During the period of the formation of the cell, the space between the membrane and the original disk is filled with fluid ; and in this a regular circulation may be seen to take place, several currents proceeding from the nucleus (or cytoblast as it is technically called) and returning to it again.

356. When the cell becomes mature, the original disk is absorbed, and no further movement of fluid is seen within the cavity; but there are some cells in which it always remains, appearing as a dark spot in their walls; and in these the circulation of fluid generally continues. This circulation may be well seen

Fig. 45. Currents in the hair of Tradescantia.

passes

in the beaded hairs of the Tradescantia Virginica, (Virginian Spider-wort), which consist of several distinct cells; at the bottom of each of these, the disk or nucleus may be seen, and several currents may be observed to proceed from it and return to it again. It is a circulation of this kind which has excited much attention in the stem and branches of the Chara, (a little cryptogamic aquatic plant) which consist only of large cells laid end to end. The fluid down one side of the stem and up the other, turning round at each extremity. If the stem (which is usually composed of a single cell, sometimes many inches long) have a thread tied round its centre, so as to separate the original cell into two, each of these will have a complete circulation of its own. A similar movement of fluid has been seen in the Frog-bit (another aquatic plant of this country) and in many others; and it is nearly certain that it takes place in every vegetable cell that exists, during some period of its growth; being only visible for a short time in some, which soon arrive at a condition little subject to change; but continuing during the greater part or the whole of life in others.

212

NUTRITION IN CELLULAR PLANTS.

357. This movement of fluid in the individual cells, is quite distinct from the general circulation which has been described in the higher plants. It is a part of the process of formation, by which the nutritious fluid that is brought to each part is converted into organized tissue. In the simple Cellular plants, where the same surface performs alike the functions of absorption, exhalation, digestion, and respiration, there is no general circulation of fluid; since each of the cells composing the whole structure imbibes the materials of its nutriment for itself, and converts them into the substance of its own tissue, or employs them in the production of new cells. These seem to be usually developed from the fluid within the parent, in the same manner as the cells of the young seed are produced from the gummy matter it contains, as just now described; and the analogy is the more close, since the membrane lining the seed may be regarded as itself a single large vesicle. The increase in size of any organ is occasioned in part by the enlargement of each individual cell, and in part by the development of new ones, which are formed in some instances between those previously existing, and in other cases (especially in the root) at the extremity only.

358. In the simplest Cellular plants, therefore, there is no necessity for any general circulation of fluid; and no other movement is seen but that which occurs in single cells.

But in the more highly-organized tribes, where the parts which receive the different kinds of food from the elements around are at a distance from each other, and from those to which the nutritious fluid must be supplied, a general circulation is required to bring them all into connection; and this is accordingly found to exist, so that every part of the structure is nourished by a fluid that has been elaborated by a system of organs, of which each is particularly adapted to a single object, whilst the actions of all are directed to a common purpose. This elaborated sap, being supplied to the growing parts of a plant, gives to them all the means of development that they can require; and they then only need the influence of light and heat, to perform their respective actions with vigour,

CHAPTER X.

OF THE SECRETIONS OF PLANTS.

359. We have seen that the elaborated sap contains the materials of the various tissues of the vegetable fabric; and an outline has just been given of what is known of the mode in which they are converted into living structure. The principal uses to man of the various kinds of these structures, will be best stated when the chief groups of plants are described, in the second division of this volume. We have next to consider a class of products, which are not of the same character; for they serve no obvious purpose in the nutrition of the plant itself, and are never converted (so far as can be ascertained) into the materials of its tissues. They usually make their appearance in the elaborated sap; but not unfrequently they are afterwards separated in some degree from it, and stored up (as it were) in a particular portion of the plant. In Animals we find a provision of a similar kind. The blood not only contains the elements of the solid tissues which are to be nourished by it, but also of fluid secretions, which are separated from it by special organs. Hence the term secretion, which means a separation or setting-apart, is derived.

360. In Animals, however, such secretions are usually destined to answer some obvious purpose, either in the system or out of it. Thus the secretion of saliva serves to moisten the food, and that of gastric juice to digest it; and in this process it is one function of the bile to assist. Again, the secretion of milk in the female for the nourishment of the young, that of poison in the venomous serpent for the destruction of its prey, that of the glutinous fluid with which the spider constructs its web, are instances of the

214

GENERAL CHARACTERS OF VEGETABLE SECRETIONS.

separation of certain ingredients of the blood, which are sent out of the body for particular objects. But Secretion in animals has other purposes;-namely to purify the blood from certain ingredients, which, if they accumulated in it, would occasion disease and even death. This is the purpose of the separation of carbonic acid by the lungs; and also, in part, of the secretion of bile, which carries off a large quantity of the superfluous carbon of the system. In the same manner, the secretion of urine carries out the superfduous nitrogen (which exists very largely in this fluid, 195. 197.).

361. Now in regard to the secretions of plants, it is very remarkable that, whilst in number and variety they much exceed those of animals, the use of them in the Vegetable economy should be much more obscure. In a few instances only are they destined to be sent out of the system; they are usually deposited in some part of it; yet they are not even separated in every instance from the nutritious part of the juices in which they are at first formed. The Secretions of plants comprehend all the peculiar products which do not form part of their tissues; thus, all the vegetable dyes, the active medicinal principles, the oils, resins, &c., and the aromatic or volatile oils, belong to this class of products. Now as the substance of which the tissues of plants are composed is everywhere almost the same, any varieties which these tissues may present, in colour, taste, &c., must be due to them; and it is from their presence that each plant derives its particular character, either as an article of food, or as furnishing products useful in medicine or the arts. The pure vegetable tissue, and the nutritious gum or starch combined with it, are nearly tasteless; and the alburnum or sapwood of trees possesses neither toughness nor colour. The former may be rendered uneatable by the disagreable taste or injurious nature of the secretions diffused through it; the latter is strengthened, and receives its peculiar colour, by the deposition in its cells and tubes, of products which have been separated from the circulating fluid, and which give to the wood a density proportionate to their amount, and to their own power of subsequently hardening.

THEIR DEPENDENCE UPON LIGHT.

215

362. The formation of these secretions is still more dependent on the influence of light, than is that of the nutritive materials themselves. Many plants, which, under the rays of a tropical sun, produce secretions of a powerful character, whether as medicines, as aromatics, or as dyes, are almost inert in colder climates, even when the amount of heat artificially given may fully equal that to which they have been accustomed. Thus, the Tobacco of Persia is universally celebrated for its peculiar perfume; and from the Roses of the South alone is it worth while attempting to obtain the powerful essential oil, which is known as Otto or Atar of Roses. This principle is advantageously employed in the growth of vegetables for the table; for, if they are reared under a diminished light, many kinds of plants may be used as food, which naturally contain secretions either unpleasant in taste or injurious in cha

Such are the Sea-kale, Lettuce, and Cichory; which are prevented from becoming rank, by heaping earth around their young shoots, or by growing the entire plant in a dark situation. The peculiar secretions, too, are not present in young plants, all whose energy seems expended in the extension of their own structure; hence those kinds which are afterwards rank poisons, may be eaten with impunity at an early period. Thus the peasants of Languedoc employ young poppies as food; and cattle do not reject noxious weeds in spring, which their instinct would not permit them to touch in summer.

363. As the special secretions of plants are formed in the elaborated sap, they will not be found in those parts to which it is not afterwards conveyed. They may generally be traced first in the leaves; but in the course of their descent, they are often separated by some particular organ, in which they are concentrated (as it were) to the exclusion of the rest. Thus many of the most powerful medicinal agents are obtained from the bark; some abound most in the roots; other products, especially resins and colouring substances, seem to be chiefly deposited in the wood; fixed oils are generally conveyed to the seeds, where they seem to be deposited for the same purpose as starch, the nourishment of the embryo; whilst aromatic oils are generally found either in the leaves,