xxi] WATER STOMATES 267 



nism employed is one which is already very general in terres- 

 trial plants, namely the development on the leaves of "water 

 pores" which are able to extrude water in the liquid state 1 . 

 These water pores, which occur singly or in groups in the 

 neighbourhood of the nerve-endings, both in submerged leaves 

 and on the under side of the floating leaves 2 , resemble large 

 stomates which remain permanently open. Beneath them, there 

 is a marked expansion of the tracheal termination of the bundle, 

 which is only separated from the epidermis by some layers of 

 thin-walled turgid cells, known as the epithem. The epithem 

 tissue is considerably developed in Dicotyledons, but less so in 

 Monocotyledons. The intercellular spaces between the cells of 

 this tissue are filled, normally, with water 3 ; the epithem is 

 believed to act as a regulator, preventing the expulsion of the 

 drop until a certain root-pressure is reached 4 . Fig. 53, p. 82, 

 illustrates the relations of the water pores and associated struc- 

 tures in the case of a floating leaf that of Pistia Stratiotes. In 

 this plant the vigorous excretion of drops of water maybe readily 

 seen, and we can scarcely doubt that, in the case of submerged 

 leaves furnished with the same mechanism, the expulsion of 

 drops also occurs, though it cannot be directly observed. 



It is a curious fact as yet unexplained that the water pores 

 of aquatics are often highly ephemeral, being resorbed and 

 destroyed while the leaf is still quite young. This occurs, for 

 instance, in Callitriche^ in which the very young leaf bears two 

 groups of water stomates at the apex (Fig. 1 63 A^ p. 268). At an 

 early stage the epidermis in the neighbourhood of the water pores 

 becomes laden with a brownish, gummy or granular material, 

 and the cells eventually die. Similar substances are apt to choke 

 up the intercellular spaces of the epithem, and the mouths of the 



1 Burgerstein, A. (1904), enumerates more than 200 genera of flower- 

 ing plants, belonging to nearly 100 families, in which the extrusion of 

 liquid water from the leaves, either by means of water pores or apical 

 openings, has actually been observed. The great majority of these are 

 land plants. 2 Schrenk, J. (1888). 



3 Volkens, G. (1883). * Gardiner, W. (1883). 



