46 THE ECOLOGY OF LEAVES 



in photosynthesis during the day accumulates in the air spaces and 

 is available for respiration during the night as well as in the day- 

 time. Carbon dioxide is usually present in water in abundance so 

 that its accumulation in the air reservoirs for purposes of photo- 

 synthesis is probably never really needed. 



In some cases, also, the air spaces serve the function of giving 

 buoyancy to the plant or plant part. For example the winter buds 

 of such plants as the duckweeds and the bladderworts, when they 

 are first formed during summer and autumn, are lacking in air 

 spaces of any considerable size. They are heavier than water and 

 in autumn they break off from the parent plants and sink to the 

 bottom. In spring large air spaces develop in the buds and when 

 these become filled with gases they cause the buds to rise to the 

 surface of the water where they develop into mature plants. Water 

 hyacinth (Eichhornia) plants are caused to float and to hold their 

 leaves just above the surface of the water by the large air spaces in 

 an enlarged portion of each petiole. A somewhat similar function is 

 performed in the case of some filamentous algae, such as Spirogyra, 

 l)y the bubbles of oxygen that become entangled in the mass of fila- 

 ments. A rain storm will wash out this oxygen and cause the plants 

 to sink to the bottom but with the return of sunlight photosynthesis 

 begins and the oxygen that is liberated soon causes the plants to rise 

 again to the surface. This phenomenon is of considerable advantage 

 to these plants since it brings them into a more favorable position 

 with respect to light and to the necessary gases as well. 



28. Structure and Arrangement of Stomata.— A stoma is a small 

 slit-like opening through the epidermis (Fig. IS). It occurs between 

 two modified epidermal cells, called guard cells, which, in surface 

 view, appear kidney-shaped and have their ends in contact. In cross- 

 section the guard cells are seen to vary considerably in different 

 kinds of plants. They are usually smaller than the other epidermal 

 cells and in some cases the walls are uniformly thickened while in 

 others they are unevenly thickened. In some guard cells the walls 

 next the aperture are thicker than those on the opposite sides of the 

 cells, while in others the outer and inner walls are thick and the 

 lateral walls thinner. Many other modifications are found although 

 the general structural features are practically the same for all 

 stomata. 



Most stomata have some capacity for opening and closing. This 

 is done automatically by the increase or decrease of the turgidity 

 of the guard cells. An increase in turgidity causes the cells to expand 



