4IO MOSSES LESS. 



By the transpiration current, then, the leaves are kept 

 constantly supplied with a solution of mineral salts derived 

 from the soil, and are thus nourished like any of the aquatic 

 green plants considered in previous lessons : by the double 

 decomposition of water and carbon dioxide a carbo-hydrate 

 is formed : this, by further combination with the nitrogen 

 of the absorbed ammonium salts or nitrates, forms simple 

 nitrogenous compounds, and from these, probably through 

 a long series of mesostates or intermediate products, proto- 

 plasm is finally manufactured. 



In this way the food supply of the green cells of the 

 leaves is accounted for, but we have still to consider that of 

 the colourless cells of the stem and rhizoids, which, as we 

 have seen, are supplied by the transpiration current with 

 everything they require except carbon, and this, owing to 

 their possessing no chlorophyll, they are unable to take in 

 in the form of carbon dioxide. 



As a matter of fact the chlorophyll-containing cells of the 

 leaves have to provide not only their own food, but also 

 that of their not-green fellows. In addition to making good 

 the waste of their own protoplasm they produce large 

 quantities of plastic products (see p. 33) such as grape 

 sugar, and simple nitrogenous compounds like asparagin, 

 and these pass by diffusion from cell to cell until they reach 

 the uttermost parts of the plant, such as the axis of the 

 stem and the extremities of the rhizoids. The colourless 

 cells are in this way provided not only with the salts 

 contained in the ascending transpiration current, but with 

 carbo-hydrates and nitrogenous compounds. From these 

 they derive their nutriment, living therefore like yeast-cells 

 in Pasteur's solution, or like Bacteria in an organic 

 infusion. 



We see then that the colourless cells of the stem and 



