APPENDAGES TO THE CUTICLE.-HAIRS.

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higher description are grown in darkness, the stomata are developed very imperfectly or not at all. Thus we have an example of the very important effects of the stimulus of light upon the vegetable structure, not only in governing its actions, but in influencing its development.

96. With the cuticle may be advantageously considered those appendages which are developed from it, as hairs, prickles, stings, &c. The leaves and stems of many plants are covered with hair, which is sometimes bristly, sometimes soft and downy, and sometimes scattered very thinly. The structure of these hairs is various. Sometimes each forms but one long cell; whilst in many other instances, every hair consists of a row of cells placed end to end, and sometimes these send off minute side branches. The cells of the hairs are usually, like those of the cuticle, destitute of fluid contents, except during the period of their formation. Their analogy with those of the cuticle is further shown by the curious fact, that many plants are hairy or not according to the circumstances in which they grow. Thus, when they are found in dry exposed situations, their stems stunted in growth, and their leaves

Fig. 24. Hairs and Glands of various kinds; a, gland surmounted by a hair; b, small gland at the top of a hair; c and e, simple hairs; d, branching hair.

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STINGS, PRICKLES.-AIR-CHAMBERS.

small, their surface is covered with hairs, as if the cells which would have otherwise formed a larger cuticle had taken the shape of hairs: whilst in damp shady places, which favour the extension of the leaves and stems, their surface is quite smooth, all the material being then required to form cuticle.

97. Sometimes the hairs are tubular and pointed, and are fixed upon minute glands in the cuticle which secrete an acrid fluid; and if but very slightly touched, the reservoir at the base is compressed, and the fluid forced up through the tube into the wound made by its pointed extremity. Such hairs are termed stings; and the Nettle affords a familiar example of them. The prickles of the Rose and other shrubs are also appendages of the cuticle, with which they are stripped off, and from which it is easy to detach them. They are thus distinguished from thorns, which proceed from the wood of the branch, and which, as will be hereafter stated, may be regarded as stunted leaf-buds. Prickles, after being once formed, and hardened by the process already described (§ 74.), undergo no subsequent enlargement; and, accordingly, if the cuticle on which they are fixed should be extended, their base is not able to expand in the same proportion, and they drop off.

98. Another interesting modification of cellular tissue is that which surrounds the spaces or cavities formed in certain plants for special purposes. Thus in the Duckweed, the leaves are provided with a set of air chambers, which give them great buoyancy; and nothing can be more beautiful than the manner in which the walls of these chambers are built up of muriform cellular tissue. In other cases, these cavities appear to be formed as receptacles for certain secreted products; and here, too, they are very beautifully partitioned off from the surrounding

RECEPTACLES FOR SECRETION.

Fig. 26. Receptacle for fluid secretion; a cavity bounded by cellular tissue.

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tissue, by a peculiar disposition of the cells. A good illustration of these is found in the rind of the orange and lemon; the odour and flavour of which are derived from the minute drops of volatile oil stored up in a vast number of these little cavities. The turpentine of resinous woods is collected in larger channels of the same description.

99. It is scarcely possible to observe the number of different forms (of which many have been left unnoticed) resulting from the varied combinations of the simple elements-membrane and fibre,—each of them probably having its peculiar function in the vegetable economy, without being struck with the simplicity of the plan by which Creative Design has effected so many marvels, as well as with the extreme beauty and regularity of the structures which are thus produced. The comparison of such specimens of Nature's workmanship as the meanest plant affords, with the most elaborate results of human skill and ingenuity, serves only to put to shame the boasted superiority of man; for whilst every additional power which is applied to magnify the latter serves but to exaggerate their defects, and to display new imperfections, the application of such to organized tissues has only the effect of disclosing new beauties, and of bringing to light the concealed intricacies of their structure.

We shall next pass on to consider the structure of the compound organs of the Vegetable fabric, and their several purposes and

uses.

CHAPTER IV.

STRUCTURE AND FUNCTIONS OF THE ROOTS.

100. The Roots are the parts of the plant on which it is chiefly dependent for the supply of the moisture which its growth requires; and they also serve to fix it in the earth. That they absorb or suck up fluid with great rapidity may be easily shown in the following manner. Take any small plant that is growing in the soil, and immerse its roots (first clearing them of earth) in a tumbler of water. If the plant be exposed to the light of day, and especially if the sun shine brightly upon it, the water will disappear very much faster from the glass, than from one exposing the same surface, and placed in the same circumstances, but without the plant. And if the specimen continue to grow and flourish, it will take up many times its own weight of water in a short period. Thus, four plants of Spearmint, grown during 56 days with their roots in water, and themselves weighing altogether but 403 grains, have been observed to take up 54,000 grains, or about seven pints of the fluid.

101. Of the water thus absorbed, a small proportion only is retained within the plant, serving as a part of its food. The greatest part of it is sent off again from the leaves, by a process hereafter to be described, termed Exhalation; and the rapidity of Absorption is in part governed by the rapidity of Exhalation. The latter is nearly checked by the absence of light; and, accordingly, plants are found to absorb but little in the night, or in a dark room. Any of the causes which will be subsequently stated to influence the latter, affect the former in a nearly corresponding proportion. The object of the introduction of a quantity of fluid

ABSORPTION IN THE CRYPTOGAMIA.

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into the vegetable system, so much larger than it retains, appears to be this; the solid mineral matters which constitute an important part of the food of the plant, are contained in the water which reaches its roots in excessively minute proportion; and it is therefore necessary, in order that a sufficient amount of these may be obtained, that all the water in which they are dissolved should be introduced. As this is by far too much for the other wants of the plant, a large part of it is got rid of by Exhalation.

102. It is only in the more perfect plants, however, that we find a restriction of this power of absorption to one particular portion of the structure. In the Sea-weeds, for example, the whole surface appears equally endowed with this faculty; and there is, therefore, no occasion for that transmission of fluid from one part to another, which is characteristic of those in which we find a stem (or something correspondent with it) bearing roots at one end, and leaves at the other. Accordingly, that which is the natural condition of the latter, is fatal to the stemless plants; for if a Sea-weed be suspended partly out of the water, the upper portion will die from drought, whilst that which remains immersed will continue to live and grow, without transmitting any of its moisture to the rest. Not unfrequently we observe the form of a stem and roots in Sea-weeds; but this is only for their attachment to rocks, or other solid substances; and the root-like portion has no special power of absorption, nor the stem of transmission.

103. In some of the Cryptogamia a little higher in the scale, however, we find a condition much more approaching that of the Flowering-plants. Thus, in the Mushroom, we observe a soft stem, which sends off fibres into the ground; and these appear to absorb by their whole surface, and to transmit the fluid they have acquired to the portion which is elevated in the air. In the Mosses, the tissue of the stem is firmer, and the rootlets which proceed from it are more woody; these not only proceed from the stem but also from the under surface of the leaves; and thus the dryness of the situations in which these interesting little plants find their subsistence is in some degree compensated for. In Ferns we