The Substance Which Is Alive in Plants 157 



is frequently the case with protective epidermal cells (figure 50, e) ; 

 or it affects only the angles, in some cells which combine water- 

 storage with strengthening (figure 50, /); and it takes various 

 other forms too many to mention. 



Furthermore, the composition of the wall is alterable both 

 physically and chemically. Cellulose is a very elastic substance, 

 and where greater stiffness than it can afford must be had, the 

 wall becomes penetrated by the far stiff er substance lignin; and 

 lignified walls are wood. Both cellulose and lignin, however, 

 allow ready passage of water, and where that would be a danger, 

 as at surfaces of plants which grow in dry air, the wall is made 

 waterproof by the formation all through its texture of a water- 

 repelling substance, called cutin or suberin; and such is the case 

 with the epidermis and cork which form the skin of plants. In 

 other cases the wall softens to mucilage on the access of water, 

 as hi Flax seeds, though the reason thereof is not perfectly clear; 

 and there are yet other such modifications of more special char- 

 acter and meaning. 



It is thus plain that cell-walls are well-nigh indefinitely plastic 

 in shape, thickening, and composition, while, moreover, any and 

 all of these features can be combined in various ways and de- 

 grees in accordance with the particular needs or functions con- 

 cerned. Furthermore, the cells are rarely isolated; but commonly 

 cooperate in large masses of similar function called tissues. 

 Masses of tissues cooperating in function, and mutually adjusted 

 to perform their work to the common advantage, form organs, 

 and organs make up the plant. 



There remains one other matter of importance about the wall. 

 Although, at first sight, it seems to shut off completely the proto- 

 plasm of each cell from that of its neighbors, minute observation 

 exhibits the presence of definite thin places perforated by very 

 fine pores which permit the passage of tiny threads of living proto- 

 plasm from one cell to another (figure 51). This continuity of 

 protoplasm from cell to cell has been found in every part of the 



