THE INTERNAL STRUCTURES OP PLANTS. 



205 



plants as much as does that of cells. The fibres of 

 basswood are about yVo" f an i ncn diameter. 



FIG. 372. 



FIG. 373. 



But the compactness of fibrous tissue depends more 

 upon the thickness of the walls of its fibres than upon 

 their fineness. Hence the density of the old heart- 

 wood of trees, where the cavities of the fibres are en- 

 tirely filled by deposited matter. Woody fibres rarely 

 exceed -% of an inch in length, while the fibres of some 

 kinds of wood are only y^-g- of an inch long. 



Tissues formed of elongated cells, particularly of 

 such cells as have tapering extremities, are called 

 prosenchyma. The cells of prosenchyma vary much 

 in length and proportions. Woody tissue is made 

 up chiefly of prosenchyma, yet some wood consist 3 

 largely of parenchyma, in which the cells have be- 

 come solid by the deposits upon their interior. At 

 first the elongated cells of wood have their ends 

 nearly square, but, as they lengthen and crowd each 

 other, they become wedge-shaped. 



The blending of cells, fibres, and vessels, in the 

 tissues of a plant, is shown in Fig. 374, which repre- 



