ii8 THE PHYSIOLOGY OF PLANTS 



in this case, too, the covering of hairs acts as a protective 

 coating, for it prevents a large amount of light from reaching 

 the leaf. 



This explains why so many desert plants, and plants growing 

 in sandy or rocky places and exposed to a powerful sun, are 

 densely covered with hairs. 



For horticultural purposes, too, we may deduce the rule that 

 plants with leaves densely coveFed with hairs require generally 

 a large amount of light and little water. We shall refer to this 

 again in dealing with the watering of plants. 



§ 21. How are the assimilating cells arranged within the leaf? 



We have mentioned in passing that the cell of chief im- 

 portance is the chlorophyll- containing cell ; its function is to 

 assimilate. These cells are so arranged in the assimilating 

 organs that the chlorophyll grains can perform the greatest 

 amount of work. The principle of increasing the superficial 

 area is carried out to the fullest extent. The assimilating 

 surface is protected by a well-developed and ingeniously 

 devised epidermal covering. 



We are now able to examine a little more closely the posi- 

 tion of the assimilating cells themselves, and we shall find 

 that here too we have a very advantageous combination of 

 various contrivances, forming a most complete arrangement 

 for ensuring the maximum of efficiency. 



Most of the leaves are horizontally expanded, so that we 

 can distinguish two surfaces, generally of different appearance, 

 one directed upwards towards the sky, and the other down- 

 wards towards the earth. This latter surface is generally of 

 a paler colour, which is due to the fact that the green cells 

 of the mesophyll are more loosely arranged and are less rich 

 in chlorophyll, and that the large intercellular spaces are filled 

 with air. 



The assimilating cells, however, which lie beneath the epi- 

 dermis of the upper side of the leaf are very closely arranged, 

 like a number of stakes driven in side by side, with their 

 narrow ends towards the upper surface, and consequently with 

 their long axis at right angles to the surface of the leaf. These 



