ELEMEXTARY STRUCTUBE. 19 



substance differin g from cellulos e, and which afterwards becomes 

 c'Ean ged"7nf o~it. 



Ifrarely happens that cellulose can be found pure in any cell- 

 membranes ; it is usually combined with various organic and in- 

 organic 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 of their age. 



Cellulose was formerly thought to be a substance peculiar to 

 plants, but it has now been found, as already noticed (page 3), 

 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 these different 

 colouring matters. When the cell-walls become thus coloured, 

 they commonly lose in a great degree their transparency. The 

 various coloiu-s which the different parts of the plant assume, as 

 the vivid tints of certain parts of the flower, 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 

 such parts are composed, but to the different colouring matters 

 which those cells contain. 

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

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

 fectly closed sac. The membrane, however, although free from ; 

 visible pores, is readily permeable by fluids. 

 f As the cell-membra ne increases in age it becomes thickened.'and 

 ' t he cells w hich are comp o sed of it increase in size. This_t]]^'f^^ ;^in£^ 

 stakes pla ce at ffrst by tSe incorporation of new maEterTn it «Sw- 

 slaLT ^c^fy r -JT^eFsti^iUi^ -, but after the cells have arrived at a de- 

 finfte size, i!^Ti1^cl^se s where lliey'^orm~pgTtgnDf~th:g^^ 



f.structu re"'of'plants, their membranes increase in thicJoiess. not 

 ho weveFas a,t fir st b y interstitial deposition, but by the succesiive 

 deposjtoTnew' mitf er upon their inner surface. This new matter 

 is generally "deposited in layers proceeding from without inwards 

 (fiffs. 34 and 35), by which the cavity of the cell is gradually 

 , diminished, and even in many cases nearly or entirely filled 

 / up. This increase in thickness may be especially observed 

 ' in the cells of the wood and inner bark, and in the hard cells of 

 the stone of the peach, cherry, and other similar fruits. This 

 thickening however of the cell-membrane, by successive layers of 

 deposit in its interior, is by no means confined to the cells of 

 the wood, or the other cases above mentioned, but it may be 



