CHAPTER II. — PLANT TISSUES. 



J 53 



and the green cells with internally folded walls, found in the 

 interior of Pine leaves, are called folded parenchyma (see Fig. 



459)- 



(2) Collenchyma, or thick-angled tissue, is closely related to 

 ordinary parenchyma, but the cells are more elongated, often 

 five or six times longer than broad, prismatic in shape, and 

 thickened at the angles. The thickenings, though often conspic- 

 uous, are usually not lignified, and the cells contain protoplasm 



Fig. 399 



Fig. 400. 



Fig. 399. — Pitted Parenchyma cells from the leaf of a Pine: magnified 300 diameters. 

 Fig. 400. — Pitted Parenchyma from the pith of the stem of Asimina triloba; magnified 

 480 diameters. 



and more or less chlorophyll. They are never found elsewhere 

 than in close proximity to the epidermis, or rarely in similar 

 relations to the endodermis, and one of the uses which they sub- 

 serve is evidently that of giving strength and resistance to these 

 portions of the plant. Every gradation is observed between this 

 and ordinary parenchyma on the one hand, and fibrous tissue 

 on the other. Sometimes it forms a continuous zone beneath 

 the epidermis, as in the petiole of the Summer Grape and of the 

 Begonia ; at others, it occurs in longitudinal bands, as in the 

 stems of the Yellow-Dock and Cow-parsnip. The tissue is illus- 

 trated in Figs. 401 and 402. 



