MORPHOLOGY 



97 



(p. 56). The fusion of the sexual cells in the process of fertilisa- 

 tion possesses special characteristics which place the process in a 

 distinct category. 



Tissues (") 



Origin and General Properties of Tissues. A continuous 

 aggregation of cells in intimate union is called a tissue. The origin 

 of vegetable tissues is, in general, attribut- 

 able to cell division. It is only in the 

 Fungi and Siphoneae that a tissue arises 

 through the interweaving of tubular cells 

 or cell filaments (Fig. 106). In such cases, 

 where the filaments are so closely inter- 

 woven as to form a compact mass of 

 cells, the tissue thus formed has the same 

 appearance as the tissues of higher plants 

 (Fig. 107). 



The mutual interdependence of the cells 

 of a tissue is manifested both by the con- Jfi 

 junction of their pits (Figs. 66, 70, 71), " 

 and by the general similarity of their wall 

 thickenings. 



The cells in a tissue may either fit no. 100. Longitudinal section of 

 closely together (Fig. 70), leaving no open- the stalk of the fructification of 

 ings or spaces, or so-called INTERCELLULAR 



SPACES may be left between the individual cells. Where cell fila- 

 ments are interwoven into a tissue, the intercellular spaces are repre- 

 sented by the openings left between the loosely-woven filaments (Fig. 

 106). In tissues resulting from cell division the intercellular spaces 

 arise subsequently, as the partition wall between two cells formed by 

 cell division is originally a simple membrane. 



Such a partition wall may ultimately split and so give rise to intercellular 

 spaces, but this only occurs after it has been thickened. The cause of such splitting 

 is to be found in the hydrostatic pressure existing within the cells, and their con- 

 sequent tendency to assume a spherical shape. The formation of intercellular 

 spaces commences, therefore, at the cell corners, where the primary wall, consisting 

 of pectinose material, becomes swollen. 



The simplest and at the same time most frequent intercellular 

 spaces are triangular or quadrangular in outline, as seen in cross- 

 section (Figs. 66 i, '74 i). In cases where special portions of 

 adjoining cells are in extremely energetic growth, intercellular 

 chambers and passages, of more or less irregular shape, may be 

 formed between them. If the growth of adjoining cells is very 

 unequal, it may lead to a complete separation of their cell walls ; 

 or the cells, or even a whole system of tissues, may be stretched and 



H 



