142 



BOTANY FOR BEGINNERS 



CHAP. 



The Organs of Transpiration. The epidermal tissue of 

 a plant is generally more or less cuticularised (p. 76), and the 

 amount of water vapour which can be given out by the epi- 

 dermis depends upon the degree of cuticularisation. In plants 

 where the epidermis is covered by a well-developed cuticle, 

 very little water vapour is given out. If the cuticle is very thin 

 or absent, as in the case of water plants, the leaves droop 

 and wither far more quickly than those with a well-developed 

 cuticle. Those parts of plants which are covered with cork, or 

 with wax, possess a protection against a too rapid loss of 

 water. The Potato is covered with a thin layer of cork, which 

 prevents loss of water through evaporation. 



The principal organs by which water vapour is transpired by 

 plants are the Stomata and the Lenticels. The stomata are 

 very small, in fact so small that neither dust nor water can pass 

 through them into the plant ; but their enormous numbers more 

 than makes up for their small size. It is calculated that a Sun- 

 flower leaf contains some thirteen million stomata, and that an 

 ordinary leaf of a cabbage may contain eleven million. 



Changes in Size of the Stomata. The stomata also regu- 

 late, transpiration by changes in their size. They open in bright 

 light, and close in darkness or in foggy 

 weather. The opening and closing de- 

 pends upon the amount of light which they 

 receive. The guard cells (p. 94) contain 

 chlorophyll, and when light shines upon 

 them the chloroplasts commence to as- 

 similate and form sugar. In this process 

 water is used up, and the cell-sap becomes 

 denser ; a current of sap is thus set up 

 from the cells in contact with them to the 

 guard cells. As more and more sap is 

 of absorbed by the guard cells, they become 

 Stoma. s, guard cell, tense, or turgid, and being fixed, they 

 s" h guard 1 cell^wlth shorten and become curved. The small 

 curved lateral wall. (S.) opening which appears between them is 



called the stoma. 



The stomata close in darkness because then the chloroplasts 

 can no longer assimilate. The sugar which has been previously 

 produced is removed by the movements of the sap, and the sap 



FIG i D'a am 

 ' 



