THE PLANT-CELL 



45 



or more chromatophores, or plastids, wanting in animal cells, which 

 are also, as a rule, less clearly delimited. The limits of the vege- 

 table cell are marked by the membrane, or cell-wall, composed 

 usually of cellulose, a carbohydrate occurring 

 but rarely in animal tissues; e.g. the mantle 

 of certain Tunicates. Owing to the presence 

 of this membrane, extensions of the protoplast, 

 except through openings in the membrane, are 

 impossible; and the tissues made up of such 

 cells are less freely motile than the tissues of 

 animals. While the cells of plants show a good 

 deal of variation in form and in the character 

 of the cell-wall, they are very seldom so changed 

 that their cellular nature is not perfectly appar- 

 ent. Where cells are isolated, as in many 

 unicellular plants and the reproductive cells of 

 the higher ones, the form of the cell is usually 

 globular or oval; but in sections of tissues the 

 cells appear more or less polygonal, owing to 

 the flattening of the walls by mutual pressure. 



The Cell-wall. The young cell-wall is deli- FIG. 30.-Piasmoiyzed 

 cate and quite colorless. As a rule, it is com- 

 posed of pure cellulose, whose chemical formula 

 is C 6 H 10 5 . Cellulose gives a characteristic 

 reaction when treated with iodine and sulphuric 

 acid, or with chloriodide of zinc. In both cases 

 the colorless walls assume a blue or violet color. The membrane 

 may later become much thicker, arid the cellulose may be more or 

 less completely replaced by other substances. The thickening of 

 the wall is either by the addition of uniform layers, or the thick- 

 enings may be unequal, resulting in characteristic sculpturing of 

 the walls, like the spines and ridges in many spores (Fig. 33) 

 and the pits, spiral bands, or reticulate thickenings on the inner 

 walls of the woody elements of many stems. 



In its normal condition, the cell-wall is strongly distended by the 

 pressure of the fluid contents of the cell. By placing a turgid cell 

 in a denser solution, e.g. a 10% solution of salt or sugar, a portion 

 of the water will be withdrawn from the cell, accompanied loy a con- 

 traction of both the protoplast and the cell-wall. This contraction 

 of the protoplast under the influence of a fluid denser than the 

 cell-sap is known as Plasmolysis. 



While the protoplasm is for the most part confined to the proto- 

 plast, it is probable that in all active tissues the cell-wall is perforated 

 by minute pores, which place the protoplasts in direct communication 

 by means of delicate cytoplasmic filaments. It seems probable, also, 



cell from the leaf of 

 Funaria hygromet- 

 rica ; some of the 

 discoid chromato- 

 phores are dividing 

 (X about 500). 



