FORMS AND STSTEMS OF TISSUES. 77 



(Fig. 65, B, ss). These intercellular spaces, which separate whole layers of cells each 

 opening to the outside in its middle by a stoma, are destined to enclose the chlo- 

 rophyll-containing tissue of these plants. The layer of cells which forms the bottom 

 of the flatly extended intercellular space, after repeated divisions vertically to the sur- 

 face, sends out protrusions upwards into the cavity ; these grow in a similar manner to 

 many filamentous Algae, divide and branch and form grains of chlorophyll, while the whole 

 of the rest of the tissue of these plants produces no chlorophyll. 



(3) The origin of resin and gum passages depends also on the formation of inter- 

 cellular passages with a peculiar development of the cells which bound them. As 

 I shall recur to other points in this structure, it is sufficient to refer to one example. 

 Fig. 66 shows 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, is also to be referred to a similar subsequent increase and correspond- 

 ing 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 contents, and by the circumstance that they excrete a peculiar sap into the 

 passage, a structure of this kind appears as a differentiated part of the tissue, which 

 is sharply marked off from its environment, and has a physiological significance of its 

 own. 



Sect. 14. Forms and Systems of Tissues. — The whole 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 

 uniform. This case is rare in the vegetable kingdom ; and it is only the simplest 

 forms that are constructed in this manner. Since in a homogeneous not differen- 

 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 arrangement of such into a 

 surface 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 body of the plant consists then of a differentiated tissue, or of different forms 

 of tissue. 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 body enclosed by the epidermal tissues, fresh 

 differentiations arise in the higher plants ; string-like arrangements of cells are 

 formed, surrounded by fundamental tissue lying between them and the epidermis; 

 these strings of tissue (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 bundles 

 and the fundamental tissue lying between them, are, however, usually not uniform 

 among themselves ; the epidermal tissue itself is often differentiated into layers of 

 different nature ; each bundle is also differentiated, but in a different manner and gene- 

 rally in a still higher degree. In this manner arise in the higher plants, in the place 



