10 ORGANOGRAPHT. 



differing from cellulose, and which afterwards becomes changed 

 into it. 



It rarely happens that cellulose can be found pure in any 

 cell-membranes ; it is usually combined with various organic 

 and inorganic substances, which modify the action of the above 

 reagents, and thus explain the differences which we find to exist 

 in the chemical properties of the membranes of the cells of 

 different plants, as well as those exhibited by the same cells at 

 different periods. 



Cellulose, as we have already seen, was formerly thought to 

 be a substance pecuUar to plants, but it has now been found by 

 several observers in the tunics of some molluscous animals, and 

 in some of the organs of the higher animals. 



b. Its General Properties and Structure. — The membrane con- 

 stituting the walls of young cells is transparent, and generally 

 colourless, although exceptions to this latter condition occasion- 

 ally occur, especially in the lower orders of plants. As the 

 cells increase in age, they frequently assume a yellow, red, or 

 brown tint, in consequence of their walls absorbing those dif- 

 ferent colouring matters, "When the cell-walls become thus 

 coloured, tliey commonly lose in a great degree their transpa- 

 rency. The various colours which the different parts of the 

 plant assume, as the vivid tints of the corolla, and the green of 

 the young bark and leaves, are not owing, therefore, to original 

 differences in the colour of the membranes of the cells of which 

 those parts are respectively composed, but to the different 

 colouring matters which those cells contain. 



The cell-membrane of young cells is very thin, smooth, and 

 free from any openings or visible pores, so that each is a 

 perfectly closed sac. The membrane, however, although free 

 from visible pores, is readily permeable by fluids, as is the case 

 with all organic membranes. Some observers, as Mulder, 

 Hartwig, and others, have described it as perforated hke a 

 sieve, but such a view, as shown by Mohl, has arisen from 

 imperfect observation. The causes which give rise to the ap- 

 pearance of pores will be described presently. It is, however, 

 quite true that in the membrane of old cells perforations do 

 occasionally occur, as in the leaves of Sphagnum, and in some 

 other cases. In the Sphagnum the holes in the cell-walls are 

 sufficiently large to allow of the passage through them of ani- 

 malcules and minute granular matters. 



As the cell-membrane increases in age it becomes thickened, 

 and the cells of which it is composed increase in size. This 

 takes place at first by the incor])oration of new matter in its 

 substance, or interstitiaUy ; but after the cells have arrived at a 

 definite size, in all cases where tlicy form parts of the pcrma- 

 ment structure of plants, tlicir membranes increase in thickness, 

 not however as in the former case by interstitial deposition, but 



