THE VEGETABLE CELL. 
391 
and Fungi, are formed of simple rows of cells; but in all the 
higher forms of plants the cells are united into tissues, the 
various forms of which have formerly been distinguished into 
two classes, Cellular and Vascular Tissue. Cellular Tissue is 
the only form of tissue found in the lower classes of Cryptogams, 
Algae, Fungi, Lichens, and Mosses, and in the rapidly growing 
parts of all plants. It consists of masses of cells which have 
undergone no material change from their original condition, 
being still closed sacs without communication with one another 
except through their permeable cell-walls, and containing proto- 
plasm, since it is only by cells in this condition that the pro- 
cesses of vegetable life and growth can be carried on. The 
44 cambium region,” in which is formed the new wood of all woody 
plants, consists entirely of cellular tissue. When the cells retain 
nearly their original form, being still roundish or elliptical, the 
tissue is termed Parenchyma ; when the cells are greatly elon- 
gated in one direction, more or less attenuated, and overlapping 
one another, the term Prosenchyma is applied to it. Vascu- 
lar Tissue , on the other hand, consists of cells which have 
undergone modification in two ways : by the thickening and 
hardening of the cell-wall, and by the fusion together of a 
number of cells. The thickening of the cell-wall seldom takes 
place uniformly, but more often in regular patterns ; and thus are 
produced the forms known as annular, spiral, pitted cells, &c., 
giving rise to the production of tissues of a similar character. 
Prof. Sachs, in his 44 Texb-book of Botany,” proposes the 
following classification of tissues : — 
1. The Epidermal Tissue. — The outer layer of cells of an 
organ consisting of a mass of tissue becomes distinguished by 
the thickness and firmness of its cell-walls, and hence usually 
by the cell-cavity being less. In the lower forms of plants 
the passage from this to the internal tissue is gradual ; in all 
the higher plants the epidermis is much more strongly differ- 
entiated. The hairs which so commonly cover the young parts 
of plants are invariably developments of particular cells of the 
epidermis, and the stomata (often erroneously called 44 breathing- 
pores,” from a mistaken view of their function) are openings 
through the epidermis to the intercellular spaces that lie 
beneath. Cork is another form in which the epidermal tissue 
occasionally develops; and in the majority of our forest-trees 
it assumes the form known as Bark. 
2. The Fibro-vascular Bundles. — The tissue of Vascular 
Cryptogams and Flowering Plants is traversed by separate string- 
like masses of tissue, which can often be completely isolated from 
the rest of the plant, and are the Fibro-vascular Bundles. Each 
separate fibro-vascular bundle consists, when it is sufficiently 
developed, of several different forms of tissue, and must there- 
