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 



II the intercellular passage, as well as by the manner of their division, by their con- 

 I 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- 

 ssue 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 different 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 Fundamental Tissue; these strings of 

 tissue, the Vascular, Fibrous, or Fibro- 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, Syste?7is 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 



