VEGETABLE HISTOLOGY AND PHYSIOLOGY. 



153 



at the angles, while they are of ordinary thickness in 



other parts. These cells occur in 



most plants of the higher orders 



and in some ferns, and are found 



in the tissues just beneath the 



epidermis. 



390. Sclerenchyma cells, some- 

 times called grit or stony cells, 

 have hardened walls produced 

 by deposition upon them of the 

 horny substance found in the 

 pits of the Cherry, Peach, and 



487, Collenchyma cells from leaf stalk 

 Plum and ,the Shells Of nutS; of Begonia; , epidermal cells; <*, collen- 



chyma cells; chl, chlorophyll grains; v, 

 SOmetimeS fOUnd in the fleshy thickened angles where these cells meet; 



p, part of parenchyma cell 



parts of the Pear. 



488, Sclerenchyma cells ; PP, canals connecting the cavity t with the outer surface or adjacent cells; 

 1, 2, 3, thickened layers; p, in B, cell walls. 



391. Epidermal cells appear in plate-like expan- 

 sions forming the outer coverings of leaves and young 

 bark ; their edges are in contact ; their boundaries are 

 either straight or sinuous ; and they are elongated in 

 the direction of growth. The edges are so firmly knit 

 together that the entire covering of one side of a leaf 

 may be removed intact. The epidermis at first is usu- 

 ally formed of a single layer, but later it is sometimes 

 made up of two or more layers (Fig. 489). 



