62 



BOTANY 



PART I 



They resemble the cells of the parenchyma in being living cells, but 



differ in the unequal thickening of their cellulose walls. This is 



localised at the angles (angle collenchyma, Fig. 63) or on the 



tangential walls (surface collenchyma). Non-living inclusions, other 



than the large vaeuole, are wanting in them. Intercellular spaces are 



absent or are very small. In spite of its high water - content 



collenchyma possesses a considerable rigidity 



against tearing owing to the thickening of the 



walls of its component elements. It at the same 



time allows and takes part in the growth of the 



organ, and may be regarded as the cartilaginous 



tissue of the plant. The distribution of the 



collenchyma is in relation to its mechanical 



functions. The extensive unthickened regions of 



the cell walls, which are further provided with 



round or elliptical pits, enable materials to be 



rapidly transported within this tissue. 



4. The Conducting Tissues. As the body of 



FIG. 62. Transverse section 

 of the sclerenchyma in 

 the leaf of Phormium 

 tenax. (x 240.) 



Fio. 63. Transverse section of the 

 collenchyma of Cucurlita Pepo. 

 (X 240.) 



FIG. 64. A collenchyma- 

 tous cell seen from the 

 side, (x 240.) 



a plant becomes larger and composed of more numerous cells, and 

 especially as more parts of it project from the soil or water into 

 the air, the need of rapid conduction of substances from one organ 

 to another (e.g. from leaves to roots and conversely) increases. The 

 movement of diffusion through the cross walls even of elongated 

 parenchymatous cells does not suffice, though facilitated by the 

 presence of pits in the wall and the complete suppression of inter- 

 cellular spaces. Special conducting tissues have therefore arisen, the 

 characteristically constructed elements of which are usually elongated 

 in the main direction of conduction, frequently present enlarged 

 surfaces for diffusion, and are further as a rule united to form con- 



