DIV. 



MORPHOLOGY 



41 



B. Origin of Tissues 



A continuous aggregation of cells in intimate union is called a tissue. 

 The origin of vegetable tissues is, in general, attributable to cell division. 

 In Hydrodidyon among the Algae a tissue is formed by the apposition 

 of free cells. In the Fungi and 

 Siphoneae a tissue arises through the 

 interweaving of tubular cells or cell 

 filaments (Fig. 37). In such cases, 

 where the filaments are so closely 

 interwoven as to form a compact 



FIG. 36. Transverse section of the sclero- 

 tium of Claviceps purpurea. (x 300. 

 After SCHEXCK.) 



FIG. 37. Longitudinal section of the stalk of 

 the fructification of Boletus edulis. (x 300. 

 After SCHENCK.) 



mass of cells, the tissue thus formed has the same appearance as the 

 tissues of higher plants (Fig. 36). The mutual interdependence of 

 the cells of a tissue is manifested both by the conjunction of their pits 

 and by the general similarity of their wall thickenings. 





C. The Cell Walls in the Tissues 



When sections of vegetable tissues are examined under a low 

 magnification the attention is attracted mainly or only by the cell walls. 

 These appear to form a network of threads something like a woven 

 tissue, and the name takes its origin from this inaccurate comparison. 



The cell walls exhibit peculiarities resulting from the connection of 

 the cells and characteristic of particular tissues. 



1. Stratification. All the septa arising in the course of cell 

 divisions in tissues are at first very thin and simple lamellae, common 

 to the two cells the protoplasts of which they separate. The cell wall 

 never remains in this condition. Even in meristematic cells it becomes 

 thickened as the membrane grows in surface extent. Thickening only 

 ceases long after the cell has reached its ultimate size. It varies 

 according to the functions taken over by the cell as part of a 

 permanent tissue, especially thick walls being found in cells which 

 contribute to the mechanical rigidity of the plant (Fig. 38). As a 



