TISSUE-DIFFERENTIATION 127 



source of food, the soil and the air. The plant can 

 take in food from the soil only in solution, and such 

 dissolved substances can transfuse only through very 

 thin cell- walls. Accordingly the roots of such plants 

 are covered with delicate threadlike processes called 

 root-hairs, each of which is composed of a single 

 thin-walled cell and is, indeed, merely an extension of 

 a cell of the skin of the rootlet itself. These root- 

 hairs are quite short-lived and are to be found 

 therefore only in the youngest root-branches. The 

 organs for taking in food from the air are the green 

 leaves. The upper and lower surfaces of the leaf 

 are usually made up of rather stiff cells, forming a 

 cuticle, in which are found numerous mouthlike 

 openings between the cells, called stomata, that lead 

 to air-spaces within the leaf. The body of the leaf is 

 made up of numerous irregular or elongate cells, 

 loosely packed together, and crowded with green 

 chloroplasts or chlorophyll bodies, the means whereby 

 the carbon dioxide of the air is fixed and converted into 

 carbohydrate (see Chapter II). 



The substances taken in through the roots, and the 

 sugars and starches formed in the leaves, are dis- 

 tributed throughout the plant-body by means of the 

 circulatory system mentioned above. In lower 

 plants with relatively undifferentiated tissues such 

 a distribution of substances must take place by direct 

 transfusion through the cell- walls themselves. As in 

 animals, however, food is not taken in continuously 

 in higher plants, at least not in the leaves. The 

 formation of sugar depends upon sunlight, and ceases, 



