THE LEAF AS STARCH FACTORY 



102. Work of the leaf. The work of food-making in the 

 leaf has already been described (see p. 53), and we may now 

 show the relation between the structure of the leaf and the 

 details of this work. The cells containing the chlorophyl must 

 get their income from the sur- 

 rounding cells or from the sur- 

 rounding air spaces. The water 

 is brought up through the ves- 

 sels of the wood (Fig. 23), and 

 it passes through the cell walls 

 by osmosis. The carbon dioxid 

 is absorbed by osmosis from 

 the air inside the leaf, and 

 this air is in direct communi- 

 cation with the outer air by 

 way of the stomates. The oxy- 

 gen given off by the cells 

 passes into the air spaces and 

 diffuses from these to the ex- 

 terior by way of the stomates. 



The skin cells are not di- 

 rectly concerned in the work 

 of starch-making ; their func- 

 tion may be described as protec- 

 tive. In addition to protecting 

 the delicate pulp cells against 



FIG. 24. Breathing holes of plants 



/, stomates, or breathing pores, on the sur- 

 face of a leaf, inclosed by the " guard cells." 

 2, section through a leaf, showing an air 

 space just inside the guard cells. Stomates 

 are found in the epidermis of twigs as well 

 as on leaves. As the stem grows tougher 

 the breathing holes become larger and more 

 irregular patches connecting the spaces be- 

 tween the cells and the outside atmosphere. 

 The roughened breathing spaces on the 

 bark are called lenticels. j, lenticels on the 

 bark of birch 



mechanical injury, they are 



even more useful in protecting 



the plant against the loss of 



water. That a great deal of water is lost by the plant through 



evaporation may be inferred from what we know about the 



evaporation of water from other wet surfaces (Fig. 24). 



103. Transpiration. The loss of water is perhaps the most 

 serious danger to which most plants are exposed, since more 

 plants die from the results of wilting than from any other one 



