FORM AND ARRANGE MENT OF VESSELS. 13 



27. The organized cells of plants appear to be the more immediate 

 seats of the various changes which constitute the functions of nutrition 

 and reproduction. In cellular plants they are the only form of elemen- 

 tary tissue produced throughout the whole of life. They absorb nour- 

 ishment through their walls, elaborate secretions, and give rise to new 

 individuals. In the newly-formed tissue of vascular plants, cells alone 

 at first exist. Fluid matters are absorbed by them, and are transmitted 

 from cell to cell by a process of transudation. The name of Endosmose 

 (ti/Soi/, inwards, p,ec.u, pa, I seek), and Exosmose (e|<a, outwards), were 

 given by Dutrochet to the process of transudation, which leads to the 

 motions of fluids of different densities placed on opposite sides df 

 animal and vegetable membranes. This process appears to be of uni- 

 versal occurrence in plants, being concerned in the movements of the 

 sap, the opening of seed-vessels, and many other phenomena. The 

 capsule of the Elaterium, for instance, opens with great force by a pro- 

 cess of endosmose going on in the cells, and such is also the case with 

 that of the Balsam. The power which cells possess of absorbing fluids 

 is well seen in sea-weeds, which, after being dried, can easily be made 

 to assume their natural appearance by immersion in fluids. It is also 

 observable in the spores of the Equisetum, the teeth of Mosses, the 

 seed-vessels of some Fig-marygolds, the Rose of Jericho (Anastatica), 

 and some Lycopodiums. 



Various organic secretions, which are necessary for growth and nour- 

 ishment, are formed by the internal membrane of cells. It is m cells 

 that the azotised and unazotised matters .ire deposited, which are after- 

 wards applied to the purposes of vegetable lit e. In them we meet with 

 the proteine compounds, albumen, fibrine, and caseine, consisting of 

 carbon, oxygen, hydrogen, and nitrogen, with proportions of sulphur 

 and phosphorus; as well as starch, gum, sugar, oil, and colouring mat- 

 ters, in which no nitrogen occurs. Some of the organic matters found 

 in plants have been artificially formed by chemical means, while others 

 have only as yet been met with in the living organism. Spiral cells 

 sometimes contain air. 



SECTION II. VASCULAR TISSUE. 

 1. FORM AND ARRANGEMENT OF VESSELS. 



28. Vascular Tissue, or AngiencJiyma (yyo?, a vessel), consists of 

 tubes whose length greatly exceeds their breadth. These may be 

 formed of membrane only, or of membrane altered in various ways 

 by deposits of fibre, or thickening matter in the interior. 



29. Woody Fibre, or Ligneous Tissue, Pleurenc/tyma (TT^IV j, a rib, 

 from its firmness), (fig. 44,) consists of tubes, or, according to some, 

 elongated cells, of a fusiform (fusus, a spindle) or spindle-like shape 

 (fig. 3), having their walls thickened so as to give great firmness. Some 



