LEAVES 



555 



leaves and by many submersed and emersed leaves and stems is deter- 

 mined by the volume of their air spaces; for example, the distended 

 petioles of the water hyacinth make the 

 plant so light that it floats with its leaves 

 high above the water. The air-contain- 

 ing bladders of various marine algae (as 

 Fucus and Nereocystis, fig. 751) serve 

 to keep these plants at levels favorable 

 for synthetic activity during periods of 

 high tide. Some fresh-water algae (as 

 Cladophora and Spirogyra) are floated 

 near the water surface by bubbles of 

 gas, which become entangled among the 

 filaments . Ceratophyllum , though usually 

 not attached, stands vertically in the 

 water by reason of the difference in 

 specific gravity between the stem and the 

 leaves, the latter being rich in air spaces. 

 During the autumn the winter buds of 

 duckweeds and bladderworts (figs. 998, 

 999), which are heavier than water, be- 

 come detached from the floating plant 

 body, and sink to the bottom of the 

 pond; in the following spring these buds 

 grow, developing large air spaces which 

 lessen the specific gravity sufliciently to 

 enable them to come again to the sur- 

 face, where they develop into ordinary 

 vegetative shoots. 



The structure of stomata. — The air 

 chambers of leaves and of young stems 

 communicate with the outside air by 

 means of stomata; as seen in surface 

 view, these organs consist usually of a 

 narrow slit, the stoma proper (though 

 the apparatus as a whole often is called 

 a stoma), flanked by two kidney-shaped 

 guard cells in contact at the ends but 

 separated along the middle (fig. 794). 



795 



Figs. 794, 795. — Stomata from 

 the leaf of an Easter lily (Lilium 

 longijlorum): 794, a stoma, as seen 

 in surface view, showing the two 

 kidney-shaped guard cells {g), which 

 enclose the stomatal aperture (5), 

 the more deeply shaded portion rep- 

 resenting the central slit; note the 

 chloroplasts in the guard cells; 6, 

 subsidiary cells ; 79s, a stoma, as seen 

 in cross section; the wall of the guard 

 cell {g) next to the subsidiary cell (b) 

 is the dorsal wall (li), the wall next 

 to the central sUt is) being the ven- 

 tral wall (/) ; the outer slit (0) is en- 

 closed between the cutinized outer 

 guard-cell ridges (r), the enlarged 

 area just below being the outer ves- 

 tibule (0') ; below the central slit is 

 the inner vestibule (/*), which here 

 opens directly into the stomatal 

 cavity (c) ; highly magnified. 



