206 BOTANY i AKT i 



small (D'0006 mm. and less) that neither dust nor water can puss 

 through them into the plant, they are usually present in such 

 enormous numbers (p. 104) and so suitably distributed that their 

 united action compensates for their minuteness. When it is taken 

 into consideration that a medium-sized cabbage leaf (Uraasicd ubT-irm) 

 is provided with about eleven million, and a Sunflower leaf with 

 about thirteen million air-pores, it is possible to estimate how greatly 

 evaporation must be promoted by diffusion through these fine sieve- 

 like perforations of the epidermis (cf. p. 221). 



The stomata also afford plants a means of REGULATING K\ .\P<H;\ 

 TION. The pores, which are the mouths of intercellular spaces, are 



Kio. 189. Stoma of lldliliOi us *//. in tran>vrrse set-lion. The ihirkiT lilies show tin- .shape assumed 

 by the guard-cells when the stoma is open, the lighter lines when the stotna is closed. (After 

 SCHWENDENER.) The cavities of the guard-cells with the stoma rinsed MV shadeil. .-aid are 

 distinctly smaller than when the stoma is open. 



surrounded by GUARD-CELLS (p. 103). As the term guard-cell sug_ 

 these cells have the power of closing the pore. THE CLOSING \\i> 



OPENING OF THE STOMATA ARE ACCOMPLISHED THROUGH CHANGES IN 

 THE TURGIDITY OF THE GUARD-CELLS OR OF THE ADJOINING El'IDKKM A I. 



CELLS. In consequence of their peculiar wall thickenings, elasticity, 

 and lateral attachment, a change of turgidity affects the size and 

 shape of the guard-cells in such a way that, on diminished turgidity. 

 they become flatter and close the air-passage, while an increase of 

 turgidity has the contrary effect and opens the pore (Fig. 189). 



In many plants the so-called subsidiary cells (p. 104) participate in various wa\ .- 

 and degrees in these processes, depending upon the special structure of the whole 

 apparatus. The opening and closing of the stomata may follow cither external or 

 internal influences ; but such stimuli affect different plants in different manners. 

 Generally speaking, the stomata begin to close on the diminution of the water- 

 supply, even before this is indicated by wilting ; they open, on the other hand, 

 when active transpiration is advantageous (in light, in moist air, etc.). The 

 quantity and quality of the substances held in solution in the nutrient water and 

 the nature of the surrounding gases react upon the stomata. The size of their 

 opening is decreased, and the quantity of water evaporated is therefore ]e.s>> in-.l 

 when more than the usual amount of nutrient salts is present, in the transpiration 



