PIV. I 



MORPHOLOGY 45 



locally, especially at the angles of the cells. At these points the walls 

 of neighbouring cells separate and INTERCELLULAR SPACES filled with 

 air arise throughout the plant (Figs. 38, 40 i). In accordance with 

 their mode of origin the smaller intercellular spaces are triangular or 

 quadrangular in transverse sections. They form a connected system 

 of narrow, branched canals (INTERCELLULAR SYSTEM) which traverse 

 the tissues in all directions. From their mode of origin by the 

 splitting of cell walls such intercellular spaces are termed SCHIZOGENOUS. 

 Unequal growth of the tissues may lead to the complete isolation of 

 cells or the formation of larger chambers or passages of more or less 

 regular form. Intercellular spaces can also arise by the dissolution 

 or breaking down of cells and are then termed LYSIGENOUS. Some- 

 times spaces, that are in their origin schizogenous, are further enlarged 

 lysigenously. Whole regions of the tissue may be stretched and 

 broken down by unequal growth. Hollow stems arise in this fashion. 

 In tissues which have arisen by a weaving together of filaments 

 (Fig. 37) the intercellular spaces are present from the outset. 



Intercellular spaces usually contain air and are of great importance 

 for the living cells forming the tissues. A single cell in water or air 

 can obtain at any time the gases, especially oxygen, which are essential 

 to its life from the surrounding medium. The life of the numerous 

 protoplasts in the tissues of a plant requires a supply of oxygen. 

 This introduction and circulation of gases in the tissues is carried out 

 by the system of intercellular spaces. 



II. KINDS OF CELLS, TISSUES, AND TISSUE-SYSTEMS 



Only in the lower multicellular plants does the tissue consist of 

 equivalent, spherical, polyhedral, and cylindrical cells (cf. e.g. Fig. 84), 

 which are similarly able to perform all the vital functions. This 

 tissue may be termed parenchyma. As the division of labour 

 between the protoplasts increases, with increase in size and progressive 

 external organisation, cells or groups of cells acquire diversity in 

 form, structure, and function. There results in the higher plants a 

 segregation of the originally uniform cells into variously constructed 

 kinds of cells, connected, it is true, by intermediate forms. Com- 

 parative study of the various organs of a plant, or of the higher 

 plants, shows that the number of these KINDS OF CELL is limited, and 



that DEFINITE FORMS OF CELLS RECUR IN THEM ALL. 



Similar cells are usually associated in groups which constitute a 

 KIND OF TISSUE. These are distinguished by the form, contents, and 

 the walls of their constituent cellular elements, and each kind of 

 tissue has its special function or functions. More highly organised 

 plants are composed of a number of kinds of tissue, but, as in the 

 case of kinds of cells, this number is small, since they recur in the 

 most diverse plants. It is not uncommon for single cells (idioblasts) 



