156 



PART II. THE INTIMATE STRUCTURE OF PLANTS. 



[31. 



"ated member are themselves elong'ated in the same direction as 

 the member ; whereas, in broad, flattened members, there is less 

 difference between the diameters of the cells ; in either case the 

 side- walls of the cells very frequently have an undulating outline, 

 so that adjoining cells fit closely together forming a continuous 

 membrane, the continuity of which is, however, interrupted in 

 certain cases by well-defined apertures, termed stomata, which 

 permit communication between the intercellular spaces of the 

 internal tissues and the external air. 



The Stomata are confined exclusively to the sporophyte-genera- 

 tion, and make their first appearance in the Moss-sporogoiiium. 

 Each stoma is an aperture bounded by two (sometimes only one, as 



in the Mosses) specialised epi- 

 dermal cells, termed guard-cells, 

 which always contain chloro- 

 plastids (Fig. 119). The aperture 

 of the stoma leads into the air- 

 cavity (Figs. 118, 120), a large 

 intercellular space between the 

 epidermis and the subjacent 

 tissue, which communicates with 

 other more internal intercellular 

 spaces. The stoma originates 

 thus : a young epidermal cell is 

 divided by a septum 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 aperture is surrounded by a single 

 annular guard-celt, as in the Mosses. The size of the aperture 

 may be increased or diminished by changes in the bulk of the 

 guard-cells ; the mechanism and conditions of this process are 

 considered in Part IV, (p. 698). 



Stomata are found on almost all sub-aerial parts of the sporo- 

 phyte of land-plants from the Mosses upward; they are especially 

 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 dorsiventral leaves (e.g. Nymphaea) they 

 are confined to the upper surface ; in radial and isobilateral leaves 



Fro. 119. Stoma of a leaf of C >mmelyna 

 ccelestis, surface view (x 300) : sp opening; 

 sz the two guard-cells surrounded by 

 several subsidiary epidermal cells. 



