Ch. Ill, 13] USES OP THE PLANT'S FOOD 97 



houses, the disastrous browning, wind-burn, and winter-kill- 

 ing of shrubs. One might think it possible to compensate 

 these difficulties by supplying water directly to leaves; 

 but leaves cannot absorb any appreciable quantity of water, 

 and such benefit as seems to follow spraying is due to the 

 check in transpiration (page 47). The spraying of plants in 

 the sun may even bring damage, because drops of water left 

 on the foliage sometimes act as small burning glasses, which 

 concentrate the sunlight, kill the protoplasm, and brown the 

 foliage in spots. 



Transpiration from leaves has another connection with 

 gardening in this way, that seedlings when transplanted con- 

 tinue to lose water; and since the absorbing roots are 

 destroyed, the plants always wilt; hence it is best when 

 practicable to cover them with boxes, etc., to check tran- 

 spiration until new roots are formed. For exactly this reason 

 gardeners remove much of the foliage of cuttings before 

 placing them in the ground to root. 



Leaves also need certain mineral matters for chemical uses, 

 involving the application of fertilizers ; and they must have 

 sufficient oxygen, which means fresh air, for their respiration. 

 These matters, however, can be considered more conveniently 

 in later sections. 



13. The Uses of the Photos ynthetic Food 



It has been said more than once in the foregoing pages 

 that the photosynthetic grape sugar made in green leaves in 

 the light is the basal food of plants and animals alike. Here 

 follows the evidence for this statement. 



The photosynthetic grape sugar and the associated pro- 

 teins move continuously from their places of formation in 

 the leaves, and pass along the veins into stems, roots, buds, 

 flowers, fruits, and other parts, every cell of which receives 

 a share thereof. Within the cells a part of the sugar and 

 proteins are chemically transformed into other substances, 

 having definite functions in the plant's economy. These 



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