590 



FARMERS' REGISTER. 



[No. 11 



of plants, and the economy of the vefjetable king- 

 dom ; but the same accuracy of weifrht and mea- 

 sure, the same statical results whicli depend upon 

 the uniformity of the laws that govern dead matter, 

 cannot be expected in operations where the powers 

 of life are concerned, and where a diversity oJ' 

 organs and of (unctions exists. The classes of 

 definite inorganic bodies, even il"we include all the 

 crystalline arrangements of the mineral kingdom, 

 are few, compared with the forms and substances 

 belonging to anin)ated nature. Life gives a pecu- 

 liar character to all its productions ; the power of 

 al traction and repulsion, combination and decom- 

 position, are subservient to it ; a Cew elements, by 

 the diversity of their arrangement, are made to 

 Ibrm the most different substances ; and similar 

 substances are produced from compounds which, 

 when superficially examined, appear entirely dif- 

 ferent. 



LECTURE in. 



ON THE ORGANIZATION OF PLANTS. OF THE 

 ROOTS, TRUNK, AND BRANCHES. OF THEIR 

 STRUCTURE. OF THE EPIDERMIS. OF THE 

 CORTICAL AND ALBURNOUS PARTS. OF 

 l^EAVES, FLOWERS, AND SEEDS. OF THE 

 CHEMICAL CONSTITUTION OF THE ORGANS 

 OF PLANTS, AND THE SUBSTANCES FOUND 

 IN THEM. OF MI^CILAGINOUS, SACCHARINE, 

 EXTRACTIVE, RESINOUS, AND OILY SUB- 

 STANCES. AND OTHER VEGETABLE COM- 

 POUNDS; THEIR ARRANGEMENTS IN THE 

 ORGANS OF PLANTS, THEIR COMPOSITION, 

 CHANGES, AND USES. 



Variety characterifes the vegetable kingdom; yet 

 there is an analogy between the ibrms and the 

 iunctions of all the ditl'erent classes of plants, and 

 on this analogy the ecientific principles relating to 

 their organization depend. 



Vegetables are living structures distinguished 

 from animals by exhibiiing no signs of preception, 

 or of voluntary motion ; and their organs are either 

 organs of nourishment or of reproduction ; organs 

 for the preservation and increase of the individual, 

 or for the multiplication of the species. 



In the living vegetable system there are to be 

 considered the exterior form, and the interior 

 constitution. 



Every plant examined as to external structure, 

 displays at least four systems of organs — or some 

 analogous parts. First, the mot. ""Secondly, the 

 trunk andbranches, or ste7n. Thirdly, the leaves; 

 and, Iburthly, \hc flowers or seeds. 



The root is that part of the vegetable which 

 least impresses the eye ; but it is absolutely neces- 

 sary. It attaches the plant to the surface, is its or- 

 gan of nourishment, and the apparatus by which 

 it imbibes food from the soil. The roots of plants, 

 in their anatomical division, are very similar to the 

 trunk and branches. The root may indeed be 

 said to be a continuation of the trunk terminating 

 in minute ramifications and filaments, and not in 

 leaves. 



When the branch or the root of a tree is cut 

 transversely, it usually exhibits three distinct bo- 

 dies: the bark, the wood, and the pith: and 

 these again are individually susceptible of a new 

 division. 



The bark, when perfectly formed, is covered by 

 a thin cuticle or epidermis, which may be easily 

 separated. It is generally composed of a number 

 of lamina? or scales, which in old trees are usually 

 in a loose and decaying state. The epidermis is 

 not vascular, and it merely defends the interior 

 parts from injury. In forest trees, and in the 

 larger shrubs, the bodies of which are firm, and of 

 strong texture, it is a part of little importance; but 

 in the reeds, the grasses, canes, and the plants 

 having hollow stalks, it is of great use, and is ex- 

 ceedingly strong, and in the microscope seems 

 composed of a kind of glassy net- work, which is 

 principally siliceous earth. 



This is the case in wheat, in the oat, in different 

 species of equisetum, and, above all, in the rattan, 

 the epidermis ofwhich contains a sufficient quantity 

 of flint to give light when struck by steel ; or two 

 pieces rubbed together produce sparks. This fact 

 first occurred to me in 1798, and it led to experi- 

 ments by which I ascertained that siliceous earth 

 existed generally in the epidermis of the hollow 

 plants. 



The siliceous epidermis serves as a support, pro- 

 tects the bark from the action of insects, and seems 

 to perform a part in the economy of these feeble 

 vegetable tribes, similar to that performed in the 

 animal kingdom by the shell of the crustaceous 

 insects. 



Immediately beneath the epidermis is the paren- 

 chyma. It is a soft substance, consisting of cells 

 filled with fluid, having almost always a greenish 

 tint. The cells in the parenchymatous part, when 

 exammed by the microscope, appear hexagonal. 

 This form, indeed, is that usually affected by the 

 cellular membranes in vegetables, and it seems to 

 be the result of the general reaction of the solid 

 parts, similar to that which takes place in the 

 honeycomb. This arranrrement, which has usually 

 been ascribed to the skill and artifice of the bee, 

 seems, as Dr. Wollaston has observed, to be merely 

 the result of the mechanical laws which influence 

 the pressure of cylinders composed of soft mate- 

 rials, the nests of solitary bees being uniformly 

 circular.* 



The innermost part of the bark is constituted by 

 the cortical layers, and their numbers vary with the 

 age of the tree. On cutting the bark of a tree of 

 several years' standing, the productions of different 

 periods may be distinctly seen, though the layer 

 of every particular year can seldom be accurately 

 defined. 



The cortical layers are composed of fibrous parts 

 which appear interwoven, and which are trans- 

 verse and longitudinal. The transverse are mem- 

 branous and porous, and the longitudinal are gene- 

 rally composed of tubes. 



The functions of the parenchymatous and cor- 

 tical parts of .the bark are of great importance. 

 The tubes of the fibrous parts appear to be the 

 organs that receive the sap ; the sells seem des- 

 tined (or the elaboration of its parts, and for the 

 exposure of them to the action of the atmosphere, 

 and the new matter is annually produced in the 

 spring, immediately on the inner surface of the 

 cortical layer of the last year. 



* My brother told me, that Dr. Wollaston, on further 

 inquiiy, relinquished the idea expressed above, and 

 adopted the common opinion regarding the manner in 

 which the hexagonal cells aie formed. — J. D. 



