44 



A PRIMER OF BIOLOGY 



stances save when exposed to sunlight, and we shall 

 see later what particular rays of light are most useful 

 to it. The cells containing chloroplasts are (with 

 certain exceptions) found only in such parts of the 

 plant as are exposed to sunlight, not only because it 

 would serve no useful purpose to develop them under- 

 ground or in deep-seated tissues, but also because, 

 apparently in most cases, sunlight is itself essential 

 to the formation of the pigment. In darkness the 

 plastid is of a pale yellow colour, familiar to every 

 one in the leaves of celery, or of grass which has been 

 overlaid by a plank of wood or other opaque object. 



The raw materials of the food of plants, as we 

 have already seen, are derived from three source?, 

 soil, water, and air. Let us consider, in the first 

 place, absorption from the soil. The soil consists 

 of a mixture of various minerals in the form of 

 granules of varied size and shape, the interstices 

 between which are filled with air and water. The 

 minerals are more or less soluble in the water which 

 circulates through these capillary channels, and 

 round each soil particle there is an extremely thin 

 film of water spoken of as hygroscopic water. From 

 the surfaces of the finest roots, for a short distance 

 behind the apices, arises a dense felt work of fine 

 hairs the root-hairs. 



These root-hairs are really elongations of the sur- 

 face cells of the root and find their way into the 

 minute crevices between the soil particles, and come 

 into intimate union with them (Fig. 17). The walls 

 of the root-hairs, where they come in contact with 

 the hygroscopic water, become swollen and mucilagi- 

 nous, and any mineral matter dissolved in the water 

 may pass through the wall of the root-hair and proto- 

 plasm lining it. This entrance takes place primarily 



