564 ECOLOGY 



volve so little oxygen and carbon dioxickthat stomata are of minor sig- 

 nificance therewith, although the gases involved doubtless pass through 

 the stomatal passageways. Stomata are unnecessary for carbohydrate 

 synthesis as well as for respiration, when the outer epidermal wall is 

 thin and composed of cellulose, as in the leaves of mosses and of sub- 

 mersed aquatics. 



Transpiration. Much the greatest movement of gas through the 

 stomatal openings is the outward movement of water vapor, known as 

 transpiration, a process that varies with the saturation deficit of the ex- 

 ternal atmosphere. It is exceptional to find an atmosphere in which 

 transpiration ceases (this condition is almost reached in some tropical 

 forests), partly because atmospheres rarely are completely saturated and 

 partly because plant temperatures commonly are higher than is the 

 temperature of the surrounding air. Transpiration through the epider- 

 mal walls (cuticular transpiration), though often significant, as through 

 the thin cellulose walls of hydrophytes and shade plants, usually is 

 much less than that through the stomata, even when the latter are most 

 tightly closed; the transpiration from an under (stoma-bearing) leaf 

 surface may be from two to five times as great as is the wholly cuticular 

 transpiration from a stoma-free upper surface, in spite of the more 

 favorable light and temperature conditions in the latter. However, 

 there is no exact relation between transpiration and the size or number 

 of stomata; a Zea leaf transpires more per unit surface with closed 

 stomata than a Hartwegia leaf with open stomata, and some halophytes 

 transpire more from the relatively stoma-free upper surface than from the 

 stoma-bearing under surface. There often are wide variations in transpi- 

 ration without corresponding stomatal movements, and the maxima of 

 transpiration and of openness of stomata do not necessarily coincide. 



The significance of stomatal structures and of guard-cell movements in the pre- 

 vention of excessive transpiration will be considered in the following section. 



Functionless stomata. Adult submersed leaves may have stomata in all phases 

 of development, some being fully formed, some resembling these except that the 

 central slit never opens, some having the air cavity clogged up with tyloses, some 

 having coalesced cutin ridges or undeveloped vestibules, while cases are known 

 where there occur only the first stages of guard-cell formation or even where develop- 

 ment ceases as soon as the stoma mother-cell is differentiated. Fully formed stomata 

 under water, though quite functionless, are not harmful, as has been thought, since 

 water does not enter through the open pores. In the sporophytes of Sphagnum and 

 Andreaea there are functionless stomata, which lack the subjacent air cavity and 

 whose guard cells do not split apart. The most tenable hypothesis concerning the 



