The Tisanat of Plants. 31 



oxygen, exist in the same proportion in water, we may say 

 tliat the substance of the cell-wall is formed of carbon and the 

 elements of water. The contents of the cell consist of the same 

 substance as the cell-wall, with the addition of nitrogen. The cell- 

 wall when young is generally colourless ; but as it increases in age 

 it assumes a variety of tints, in some cases of red, in others of 

 yellow, or of brown, just as we may notice that when a tree has 

 first come out in spring, its leaves are of a more or less pale 

 yellow green colour, but that in autumn they grow red or brown. 

 'J'he green colour of leaves is caused by small granules in the 

 cells, which are called chlorophyll. 



Now as to the fluid which is contained between the cell-wall 

 and the nucleus. This fluid is in a continual state of motion in 

 certain currents. The cause of this motion has not been yet dis- 

 covered, nor is it known what office it performs, as there is little 

 doubt that it is not connected with the circulation of the water 

 which the plant sucks up from the earth. That there is circulation 

 there is no doubt, though it cannot be seen without a very high 

 magnifying power. We now come to, perhaps, the most interesting 

 question about cells; viz., how does the sap which the plant im- 

 bibes from the eanh circulate through the plant .' The cell-wall of 

 each cell is at any rate at first quite entire, nor is it composed of a 

 porous substance through which the sap could freely circulate. The 

 sap, however, passes through by a process which has been called 

 endosmosis. The law which governs it is this: — When two fluids 

 of unequal density are separated by an organic membrane, which 

 is permeable, though not necessarily porous, there is a tendency to 

 equalise the density, from a formation of a double current, i.e., by 

 some of the lighter fluid passing into the denser, and the denser 

 into the lighter. Hence, when the root of a plant sucks up water, 

 some of the water will pass into the denser liquid of the cell-con- 

 tents, and hence the water will circulate by degrees throughout the 

 whole plant. But there is another difficulty which arises, and 

 which would at first seem to prevent the cii'culation of the water. 

 The substance of the cell-wall as it grows older becomes con- 

 siderably thicker by the following process. A thickening matter is 

 deposited on the inside of the cell-wall called lignine, and thus, 

 when the cell has been formed some time, almost fills it up. In 

 some cases this gets quite hard, as we may see in the case of the 

 tissues of the stone of any kind of plum. This thickening, were it 

 to take place all over the cell-wall, would effectually prevent the 

 circulation of water ; but the fact is that it never does take place 

 all over, small places being always left uncovered, which look like 

 little dots. Often at these ])laces the cell-wall breaks, and thus 

 are formed regular pores. It is chiefly owing to this thickening 



