31] 



CHAPTER II. THE TISSUES. 



157 



the distribution of the 

 sfcomata is uniform on all 

 sides ; they are wanting 

 in submerged leaves, and 

 are always absent from 

 roots. 



A peculiar form of stoma is 

 found in some plants, known as 

 a water-stoma (Fig. 121). It con- 

 sists of two large, almost sphe- 

 rical, guard-cells which cannot 

 alter their form so as to close the 

 aperture. Water- stomata occur 

 on the leaves of some of those 

 plants (e.g. Alchemilla, Cras- 

 sula, Ficus, Saxifraga, Colo- 

 casia, Papaver, Tropaeolum) 

 which excrete water in the form 

 of drops ; they are situated over 

 the termination of the vascular 

 bundles on the margins or at 

 the apex of the leaf ; when 

 chalk-glands are present (p. 

 137), water- stomata are de- 

 veloped in connexion with them 

 (see Fig. 100). 



In some plants (e.g. Grasses) 

 which excrete drops of water, 

 the water escapes through fis- 

 sures in the epidermis of the 

 leaf. 



The epidermis of the 

 submerged shoots of 

 water-plants differs from 

 that of land-plants in that 

 it is not cuticularised, in 

 the absence of stomata, 

 and in that its cells fre- 

 quently contain chloro- 

 plastids. 



The epiblema of the 

 subterranean root is com- 

 monly known as the pili- 



Fro. 120. Epidermis with stomata, from the 

 lower surface of the leaf of Hellel>orus fcetidus: A 

 in section; .B surface view (x 300); e epidermal 

 cells; c cuticle; I thickenings of the external 

 wall; / foils of the lateral walls; s stoma; ss 

 guard-cells; sp aperture; a air-cavity; cl meso- 

 phyll. 



Fro. 121. Water-stoma from the margin of the 

 leaf of Tropceolum majus, with surrounding epider- 

 mal cells. (After Strasburger : x 240.) 



