156 



PART II. THE INTIMATE STRUCTURE OF PLANTS. 



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gated member are themselves elongated 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-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 mechanism and conditions of this process are 

 considered in Part IV. 



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 



FIG. 119. Stoma of a leaf of Commelina 

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

 sz the two guard-cells surrounded by 

 several subsidiary epidermal cells. 



