108 



PART II. ANATOMY AND HISTOLOGY. 



[30 



leads into the air-cavity (Fig. 88), a large intercellular space 

 between the epidermis and the subjacent tissue, which communi- 

 cates with other more internal intercellular spaces. The_stoma 

 originates thus : a young epidermal cell is divided 



a septum 



r 



into two halves, each of which becomes a guard-cell; the septum 



then gradually splits into 

 two, and thus the aperture 

 between the guard-cells is 

 formed ; when the septum 

 does not quite reach across 

 the mother-cell, the aper- 

 ture is surrounded by a 

 single annular guard-cell, 

 as in the Mosses. The 

 size of the aperture may 

 be increased or diminished 

 by changes in the bulk of 

 the guard-cells ; the me- 

 chanism and conditions of 

 this process are considered 

 in Part III. 



Stomata are found on 

 almost all sub-aerial parts 

 of the sporophyte of laud- 

 plants from the Mosses 

 upward ; they are especi- 

 ally abundant on leaves 

 (as many as 600 to the 

 square millimetre"), and, in 

 dorsiventral leaves, more 



particularly on the lower (dorsal) surface, but in. floating dorsi- 

 ventral leaves (e.g. Nymphsea) they are confined to the upper 

 surface ; in radial and isobilateral leaves the distribution of the 

 stomata is uniform on all sides ; they are wanting in submerged 

 leaves, and are always absent from roots. 



FIG. 83. Epidermis with Rtonmta, from the 

 lower surface of the leaf of Helle'ioi-tis fixtidus: A 

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

 cells; c cntiele; I thickenings of the external 

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

 guard-cella ; p aperture; a air-cavity; cl meso- 



Aj>eculiarform_of stoma is found in some plants, known as a icater- 

 stoin*t_(~FiK.7&). In consist^ of two large, almost sphericalTguard-cells 



which cannot alter their form so as to close the aperture. Water-stomata 

 -K'cur on the 1, .fives of some of those plants (e.g. Alchemilla, Crasnla, Flcus, 

 Saxifraira, Colocasia, Papaver, Tropreolum) which excrete water in the 

 form of drops; they are situated over the termination" of the vascular 



