124 PHYSIOLOGICAL BOYANY. 



growing, it takes new and various forms, which, on account of 

 the two causes that control them, may be classed as inherent 

 and casual. The inherent forms of the cell, or those depending 

 on its own laws of growth, may be referred to three general 

 types (1) spheroidal, like Pollen grains, the red Snow-plant, 

 the cells of leaf-tissue, etc., varying to oblong, or lobed, or stel- 

 late ; (2) cylindrical, or tube-form, as most wood-cells are ; (3) 

 tabular or flattened, as the cells of the epidermis. 



S71. The casual forms result from external pressure as of 

 cells crowding against cells, in stems or pith. In this way 

 spheroidal cells may become cubical, 8-sided, 12-sided, etc., tubi- 

 form cells, prismatic, and tabular cells 4-angled, hexagonal, etc. 



372. In magnitude the plant-cell varies from yj-g- to -g-J-g- of an 

 inch in diameter. The cells of Elder pith measure about ^ 

 inch ; cells of parenchyma (leaf-tissue) about j^-g- ; consequently, 

 64,000,000 of them would occupy only one cubic inch. The 

 cells of cork are computed to be 10 * 00 inch in diameter 1000 

 millions to a cubic inch. But the length of some cells is more 

 considerable. Wood-cells measure -5^ inch ; bark-cells, as Flax, 

 Hemp, nearly -j- inch ; the cells of some plant hairs, an inch or 

 more. 



373. The wall of the new cell consists of two layers; the 

 outer one a firm, colorless membrane, made of celhdose, the in- 

 ner a plastic, gelatinous layer applied to the outer, and chiefly 

 concerned in cell-life and multiplication. This is called the 

 primordial titricle. It is best seen when treated with a weak 

 solution of nitric acid, iodine, or alcohol. It thus becomes col- 

 ored, contracts, and lies loose in the cell (472). 



374. 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 under the highest magnifier. 



375. A secondary layer 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 : 



376. Wood-cells, which are finally filled up by the repetitions 



