6 MORPHOLOGy OF THE CELL. 



thickness; but this thickening is usually not uniform; particular spots remain thin, 

 in others the thickened membrane projects internally or externally in ribbon-shaped 

 prominences, spines, or knobs. In the substance of the cell-wall itself differences 

 also manifest themselves, which give it greater firmness, elasticity, or hardness, 

 or, on the other hand, greater softness or pliancy. The protoplasm may, in 

 these processes, decrease more and more, until at last it forms an extremely thin 

 membrane, applied so closely to the cell-wall that it does not become visible till 

 contraction takes place ; after the completion of the growth of the cell-wall it may 

 even entirely disappear. But in many other cases the protoplasm increases with 

 the increase in volume of the cell ; it forms a- thick-walled sac, the substance of 

 which is endowed with constant motion, while filiform or ribbon-shaped strings 

 of protoplasm often traverse the sap-cavity. In those cells which appear externally 

 green, certain portions of the protoplasm become separated, and assume a green 

 colour; these particles, the Chlorophyll-bodies, may have the form of bands, stars, 

 or irregular masses ; but they usually form numerous roundish granules, and are 

 always parts of the collective protoplasm -mass of the cell. Sometimes, mixed with 

 the green colouring-matter which tinges them, are pigments of other colours, red, 

 blue, or yellow (as in the Florideae, Oscillatorieae, and Diatomacese) ; or the chlo- 

 rophyll-granules assume, through changes in their colouring-matter, other tints, 

 mostly yellow or red. Colouring-matters may also be dissolved in the cell-sap. 

 The other chemical compounds, which are formed in extremely large num|Ders in 

 the cell, are mostly dissolved in the cell-sap; but many of them assume definite 

 forms; thus arise granules of fat, drops of oil, and frequently true crystals or 

 crystal-like bodies. One of the commonest granular compounds present-in almost 

 all plants, with the exception of Fungi and some Algae and Lichens, is starch, the 

 grains of which often accumulate in the cell in quantities greatly exceeding all other 

 substances. 



Cells of the most perfectly developed form are found in certain families of Algae, 

 the Conjugatse, Siphoneae, and Diatomaceae. Since in these cases the same cell unites 

 in itself all vegetative functions, and at the same time a many-sidedness presents itself 

 in the vital phenomena, the whole cell attains a high degree of differentiation ; the 

 separate parts — the cell-wall, the protof)lasm, and the bodies enclosed in it — show a 

 variety of structure which does not occur elsewhere concurrently in the different parts 

 of one and the same cell. In addition to this, the same cell has in these cases often to 

 go through the most diverse metamorphoses, so that besides its manifold development as 

 to size, it also undergoes a series of transitory changes of form. Hence these types 

 of Algae are of great importance for an accurate comprehension of the nature of the 

 cell. (See Book II, Algae.) But these cells are also remarkable in this respect, — that, 

 after they have attained their highest grade of development, they are still able to divide 

 and to multiply ; sooner or later the protoplasm can abandon the cell-wall, contract, with 

 all its contents (starch, oil, chlorophyll, &c.), expel the water of the cell-sap, and form 

 eventually a new cell. 



We may pass over the innumerable intermediate forms, and turn our attention to 

 the other extreme, namely, to those plants of which each usually consists of thousands 

 or even millions of cells, as is the case with Vascular Cryptogams and Phanerogams, 

 and in which the different parts of the plant undergo an entirely different morpho- 

 logical development, and are adapted to different functions for the support of the 

 whole. Here we find that certain cells never attain their full development ; they 

 remain constantly in the immature condition which is represented in Fig. i, A; these 



