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 wiih 
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, Oscillatoriese, and Diatomaceae) ; 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 numbers in 
the cell, are mosdy 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-Hke 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 Conjugatae, Siphoneae, and Diatomaceae. Since in these cases J;he 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 protoplasm, 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, Algas.) 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. A\ these 
