OF THE VEOBTABLB OKLL. 



131 



657, Section of the rhizome of Blood-root, o, a. A bundW 

 of wood-cells. The shaded cells contain the color. 



(1) spheroidal, like pollen grains, the red snow-plant, the cells of leaf- 

 tissue, etc., varying to oblong, or lobed, or stellate ; (2) cylindrical, or 

 tube-form, as most wood-cells are ; (3) tabular or flattened, as the cells 

 of the epidermis. 



642. The casual forms 

 result from external pres- 

 sure, as of cells crowding 

 against cells, in stems or 

 pith. In this way spher- 

 oidal cells may become cu- 

 bical, 8-sided, 12-sided, 

 etc ; tubiform cells pris- 

 matic, and tabular cells 4- 

 angled, hexagonal, etc., in 

 outline according to the 

 original pattern. 



643. In magnitude the plant 

 eell varies from -pj-j to jj^ of 

 an inch in diameter ; the more 

 cotnmon size is about jjj- inch. 

 Tho cells of older pith measure 

 about 2^5 inch; those of pa- 

 renchyma (leaf-tissue) about ^^j consequently, 64,000,000 of them would occupy 

 only one cubic inch. The cells of cork are computed to be y^Vir '■^'^'^ ™ diameter — 

 1000 millions to a cubic inch. 



644. But the length of some cells is much more considerable. Wood-cells 

 measure -g^ inch ; bark cells, as flax, hemp, nearly J inch ; the cells of some plant- 

 hairs an inch or more. 



645. The wall of the new cell consists of two layers ; the outer one 

 a firm, colorless membrane, made of cellulose, the inner a plastic, gelat- 

 inous layer applied to the outer, and chiefly concerned in cell-life and 

 multiplication. This is called the primordial utricle. 



646. It is best seen when treated with a weak solution of nitric 

 acid, iodine, or alcohol. It thus becomes colored, contracts, and lies 

 loose in the cell. 



647. The cell wall is easily permeated by fluids flowing in and 

 out. It must, therefore, be regarded as porous ; although it appears 

 perfectly entire even flnder the highest magnifier. 



648. A secotjdary later is subsequently added to the outer layer, 

 between it and the primordial utricle, as if to strengthen it. This new 

 layer is seldom entire, but perforated and cleft in a great variety of 

 patterns, leaving certain points or parts of the cell-wall still bare 

 and discernible by their transparency. Hence the following varieties 

 of cells : — 



