50 THE ELEMENTARY STRUCTURE OF PLANTS. 



64. All the clifFerent kinds of tissue that enter into the composi- 

 tion of the plant have now been described, and all (excepting the 

 doubtful latex-vessels) referred to the cell as their original. Every 

 plant, or each organ, consists at first of one or more cells of proper 

 cellular tissue. In many of the simpler vegetables, the cells multi- 

 ply in this primitive form solely ; and the fully developed plant con- 

 sists o^ parenchyma alone. But in all plants of the higher grades, 

 some of them early assume the forms 'of wood-cells and of ducts. 

 These modified cells always lie vertically in, or conspire to form, 

 bundles or cords that run lengthwise through, the stem or other 

 organ they occur in. They are associated with each other, and 

 together make up the woody parts, as in the wood proper, in the 

 liber or inner bark, and in the fibrous framework of the leaves. 

 Although the various kinds exhibit transitions through every man- 

 ner of intermediate forms, the whole, taken together, compose tissues 

 which are almost always manifestly different from the parenchyma 

 in which they are imbedded. It is convenient, therefore, to give 

 them a general name, and to denominate them, from their position, 

 the Vertical or Longitudinal System, or, from their nature, the 

 Fibro-vascular or Woody System ; in contradistinction to the Hori- 

 zontal or common Cellular System of the plant, consisting of paren- 

 chyma alone. 



65. Iiltcrcellnlar System. The only exception to the statement 

 that all the vegetable tissues are formed of cells, is that of the 

 so-called vessels of the latex, which, according to the view now best 

 supported, ai"e a secondary formation, resulting from the transuda- 

 tion of peculiar assimilated matters into the interspaces between the 

 cells ; and are therefore rather to be classed with other receptacles, 

 canals, or intervals that are found among or between the cells. 

 Some of these are accidental, or at least ai"e irregular and indefi- 

 nite : such are the Intercellular Spaces or Passages, left 

 when the cells are not in contact throughout. Of the same char- 

 acter are the larger and irregular spaces in the lower stratum of 

 the tissue of most leaves (Fig. 7 and Fig. 221), and wliich form 

 irregular winding passages through which the air, admitted through 

 the stomates (70), freely circulates. 



^&. Air-Passages, however, are not always so irregular. The stalks, 

 and often the foliage also, of aquatic and marsh plants generally 

 abound with regular air-channels, of much greater diameter tlian 

 the cells of the tissue. These air-passages are symmetrically ar- 



