294 MANUAL OF BOTANY 



much chemical alteration. The nature of the middle layer 

 varies ; in adult cells which have not become lignified or other- 

 wise altered, it consists of a compound of pectio acid and calcium. 

 This layer can be distinguished between even very young cells, 

 by appropriate reagents. 



The cell-wall is often found to be impregnated with wax, 

 which in some cases is so plentiful as to form a layer of particles 

 on the external surface. This can be seen in the bloom of cer- 

 tain fruits, such as the Plum and the Grape ; also on the leaves 

 of the "Wax Palm. 



Certain modifications of celliilose also occur, unmixed with 

 any of these derivatives. The walls of the hyphse of Fungi afford 

 an example of these ; the cellulose, sometimes oeiQ.e& fungus cellu- 

 lose, does not turn violet when treated with iodine and sulphuric 

 acid. In the walls of certain cells, such as the bast of Lycopo- 

 dium, the endosperm of the Pasony, and the cells of the cotyle- 

 dons of some of the LeguminosEe, a variety of cellulose is found 

 which stains blue with iodine alone. 



Chemically regarded, the ceU-waU is thus seen to be very 

 complex in composition. Originally it consists mainly of 

 cellulose, with a small admixture of pectoses ; as it grows older 

 this simplicity disappears, and it is found to be a mixture of 

 those primary constituents with various products of their decom- 

 position, which are present in very varying jjroportions, or with 

 other materials which have been absorbed by it and deposited in 

 its substance. 



The Cell-contents. — Besides the protoplasm or living mate- 

 rial present in the cell, very many other substances are found 

 there which are extremely variable in nature and amount. All 

 these are to be associated in some way with the vital activity of 

 the protoplasm. As the latter is practically stationary in the cell 

 and carries on its life-work there, it is evident that the sub- 

 stances necessary for its nutrition must be brought to it ; and as 

 these can only pass readily from cell to cell by means of the 

 water which saturates the living parts, they must be in solution. 

 The water which is always present in the cell is known as the cell- 

 sap. It may be equally distributed through the protoplasm as 

 in young cells, or it may occupy also spaces in the latter which 

 we have already described as vacuoles. It is never present as 

 pure water, but always has various chemical substances dis- 

 solved in it. Besides containing the inorganic substances absorbed 

 from outside, it also has in solution in it many bodies which 

 have been made by the acti\ity of the protoplasm ; and as the 



