106 WATER-STOMATA [CH. 



of the teeth in aerial leaves, where their function the 

 exfiltration of water under pressure comes into play 

 when circumstances are unfavourable to the action of the 

 ordinary stomata, for instance, when the atmosphere is 

 saturated. Examples occur in Fuchsia globosa, Primula 

 sinensis, Elder, Cyclamen, Saxifrages, Ash, Elms, Willows, 

 Prunus Padus, Plane, Corylus, &c. Water-pores occur in 

 some cases as temporary structures on young leaves, and 

 are functional before the true stomata are completed, but 

 shrivel up later e.g. Tropceolum, Colocasia, Aconitum, &c. 

 In the Aspen they occur on the leaves of seedlings, 

 suckers and stool-shoots, but are said to be absent from 

 the more motile ordinary leaves. 



In some cases the water-pores are in groups, and may 

 be of two sizes, e.g. a larger pore surrounded by several 

 smaller ones (e.g. Tropceolum Lobbianum). 



While the essential structure of water-pores is like 

 that of stomata, their larger size, more curved guard-cells 

 which are also incapable of opening and closing, and their 

 relations to the tissues beneath are important points of 

 difference. They lie over the ends of vascular bundles, 

 the latter being usually dilated into a group of thin- 

 walled gland-like cells, through which the water is filtered 

 and exudes as drops at the pores. It should be mentioned, 

 however, that such bundle-ends can exfiltrate water without 

 the coexistence of definite water-pores; and in Grasses, 

 for instance, drops of water escape through mere fissures 

 in the epidermis, while in other cases ordinary stomata 

 are utilised when the pressure of water is great. 



It has been stated that as many as eighty-five drops 

 of water per minute may exude from the large water-pores 

 at the tip of the leaf of Colocasia, and that more than 

 22 c.c. have been collected in a single night. 



An essential difference between this liquid water 



