X] FUNCTIONS OF STOMATA 105 



to prevent this among other factors the waxy bloom, 

 cuticular papillae and rodlets, hairs, &c., or the restriction 

 of the stomata to depressed areas, grooves, &c., or on the 

 lower surface of inrolled leaves protected by hairs, &c., all 

 serve this end. 



It is stated that considerable differences exist in the 

 power of opening and closing their stomata in different 

 trees. The Aspen, essentially a tree of moist situations, 

 is, according to Stahl, able to live on dry soils because it 

 can close its stomata so completely that its leaves adapt 

 themselves to the altered conditions ; whereas most 

 Willows are unable to do this, and suffer accordingly if 

 planted in dry situations, and similarly the Birch and 

 Alder are deficient in this regulating action. 



That the stomata exert a powerful influence on as- 

 similation, according as they are wide open and promote 

 transpiration and the free access of gases, seems clear from 

 Stahl's experiments with Lime, Honeysuckle, Lilac, Elder, 

 &c., where he found that with closed stomata and drooping 

 leaves no more starch was formed in the light. 



Evergreen leaves e.g. Yew, Box, Ivy close up their 

 stomata in autumn and thus husband their water-supplies 

 in winter. 



The stomata and pores so far described are adapted 

 especially for the passage of water-vapour, and gases in 

 general, but many plants are known to possess much 

 larger apertures, similarly developed and constructed but 

 adapted especially for the passage of liquid water when 

 such occurs in excess in the tissues beneath. These are 

 the so-called water-stomata or water-pores. 



They are by no means uncommon on the submerged 

 parts of aquatic plants, where they evidently replace the 

 true stomata which such organs lack ; but their most 

 interesting occurrence is at the margins, apices and tips 



