FORMATION OF THE COMMON WALL OF CELLS. 
79 
(3) The origin of Resin-, Gum-, and Latex-passages depends also on the formation 
of intercellular spaces with a peculiar development of the cells which bound them. 
As I shall recur again to these structures, it is sufficient here to refer to one example. 
Fig. 66 represents passages of this kind in the transverse section of young portions of 
the stem of the ivy. Conditions such as B, C show clearly that the intercellular 
space arises by the parting of four or five cells ; and that these latter, distinguished 
by their turbid granular contents, increase by division. The formation of the much 
wider passages D, E must also be referred to a subsequent increase and growth 
of the cells which surround the passage. By the growth of the cells which bound 
the intercellular passage, as well as by the manner of their division, by their con- 
tents, and by the circumstance that they excrete a peculiar sap into the passage, 
a structure of this kind becomes a differentiated part of the tissue, which is 
sharply marked off from its environment, and has a physiological function of its 
own. 
Sect. 14. Forms and Systems of Tissues. — The entire mass of the cell- 
tissue which forms the body of a plant may be uniform or not; in the first case 
the cells are all similar to one another, and their modes of union everywhere the 
same. This case is rare in the vegetable kingdom ; and it is only the simplest 
organisms that are constructed in this manner. Since in a homogeneous undifferen- 
tiated tissue all the cells are alike, their union into a whole is physiologically and 
morphologically of very subordinate importance, because each cell represents the 
character of the whole tissue. Hence it not unfrequently happens in these cases 
that the cells become actually isolated, and continue their life singly; and such 
individuals are termed Unicellular Plants. Only a little higher are those which 
consist of an unbranched row of perfectly uniform cells, or of an aggregation 
of such into a plate or mass. When numerous and densely crowded cells 
form a mass of tissue, then it is usually the case that difierent layers of tissue 
develop differently; the plant then consists of differentiated tissues. In general 
their arrangement is determined by the fact that the whole mass of tissue has 
a tendency to become definitely bounded on the outside, so that there arises 
a differentiation of outer layers of tissue from the inner mass. But in the 
interior of the mass enclosed by this Epidermal Tissue fresh differentiations arise 
in the higher plants ; string-like arrangements of cells are formed, separated 
from one another and from the epidermis by Fimdamental Tissue ; these strings of 
tissue, the Vascular, Fibrous, or Fihro- Vascular Bundles, usually follow in their 
longitudinal course the direction of the most vigorous growth which immediately 
precedes their differentiation. Not only the epidermal layer, but also the vascular 
bundles and fundamental tissue, are however usually themselves differentiated ; 
the epidermal tissue into layers of different nature ; the bundles also exhibit 
differentiation and generally in a still higher degree. In this manner arise in the 
higher plants, instead of different layers, Systems of Tissues. We thus usually 
find an Epidermal System, a Fascicular System, and the System of Funda- 
mental Tissue lying between them (Fig. 67). But whenever a differentiation of 
tissues of this kind occurs in a plant, it only takes place progressively; originally 
the whole mass of a growing portion of the plant (stem, leaf, or root) consists 
of a uniform tissue out of which, by diverse development of its layers, these 
tissue-systems have their origin. This tissue of the youngest parts of plants 
