CHAPTER II.— PLANT TISSUES. 



155 



toward the condition of stone cells. In sclerotic tissue the cell- 

 wall is usually lignified, and the thickening is in layers, present- 

 ing the appearance of concentric rings. There are also delicate 

 tubes or pore-canals radiating from the cell-cavity to the outer 

 portion of the cell-wall. 



These are the cells which give the great hardness to the 

 outer coats of many seeds, and the shells of nuts. They consti- 

 tute the gritty particles that occur in the flesh of some fruits, as 

 that of the Pear, and they occur in the pith of the Apple, Men- 

 ispermum Canadense, and Hoya carnosa, in the cortex of the 

 root of the Dogbane, etc. They, are, in fact, seldom entirely 



absent from the more highly 

 organized plants. See Fig. 



4°3- 



(4) Epidermal Tissue. 

 The epidermis constitutes 

 the primary covering of the 

 plant. It usually consists of 

 one, but sometimes of two 

 or three layers of cells. The 

 component cells are so 

 placed with reference to 

 each other, that no inter- 

 cellular spaces exist except- 

 ing where the stomata and 

 water-pores occur, and com- 

 monly they have that por- 

 tion of the cell-wall which faces outward, considerably thickened 

 and cutinized. When viewed from the upper or under surface, 

 they often appear sinuous or irregular in outline, but sometimes 

 they are straight-sided and regular. In many plants, also, they 

 are somewhat elongated in the direction of the length of the 

 organ. This is especially true of the cells which cover the 

 veins on the under surface of leaves. In such cases they are 

 not only considerably elongated but often, also, oblique-ended 

 or fusiform. Different forms of epidermal cells are shown in 

 Figs. 405 and 406. 



Stomata are modified epidermal cells, usually crescent-shaped, 

 occurring in pairs, and so placed that the concave sides face 



Fig. 404. — Portion of transverse section 

 of exterior part of stem of Menispermum 

 Canadense, showing cuticle, c, greatly thick- 

 ened, and sub-lying collenchyma tissue. 

 Magnified about 375 diameters. 



