THE LEAF 213 



of Orchids make the same provision for long periods of drought 

 in using their enlarged stems or pseudo-bulbs as storehouses 

 of nutrimei;;t and water, upon which the plants feed in times of 

 need. 



And not only are these plants thus enabled to store up water 

 against the time of drought, they also keep this precious fluid 

 under a thick, leathery skin, through which only very little 

 moisture can escape. 



Another very common means of protection against too great 

 a loss of moisture by transpiration is the hairy covering of their 

 leaves. The hairs keep a layer of quiet air within their spaces 

 and thus prevent the access of new, unsaturated air which would 

 dry up the leaves in a short time. (See Anaphalis, p. 71). 



Many leaves also are shiny, and reflect a great deal of the 

 heat which would otherwise raise their temperature, and so in- 

 crease the activity of evaporation {cf. Mango, page 25). 



And, last but not least, most plants have the wonderful power 

 of closing their stomata as soon as the amount of water coming 

 up from the roots becomes scarce. 



Plants with such contrivances to keep down the loss of water 

 to a minimum are termed xerophytes. 



2. Assimilation of Food. 



I. x\bsorption of Mineral Salts by the Roots. 



In the preceding chapter we have seen that plants need a 

 constant supply of water. The water taken up by the roots 

 contains substances from the earth dissolved in it. Which are these 

 substances? If we burn a plant carefully, an ash remains. The 

 minerals constituting the ash, must have been the substances 

 absorbed by the plant. Chemists tell us that the principal 

 minerals found in the ash of plants are sulphur, phosphorus, 

 potash or soda, lime, silex, magnesium and iron. Other substances 

 that formed part of the body of the plant, but were dissipated 

 by the heat, are carbon, water, and nitrogen. 



All the substances named above, excepting carbon, form the 

 mineral food absorbed by the root. We prepare a solution of 



