554 ECOLOGY 



in cell shape without conspicuous lacunar development in the palisade layers. 

 Two theories have been suggested, one that lacunae are a, reaction to low oxygen 

 pressures, the other that they are developed where transpiration is weak. The 

 oxygen theory has been employed only in connection with the great develop- 

 ment of lacunae in water plants, especially where aerenchyma is formed. The 

 evidenceforthis theory is slight; differences in oxygen pressure quite fail to account 

 for the sudden change from lacunar to compact tissue at the water line (as in the 

 leaf of Nymphaea and in stems with aerenchyma). Almost without exception 

 lacunae vary inversely with the transpiration, the largest air chambers being in the 

 water where transpiration is reduced to zero. Elsewhere palisades have been seen 

 to vary directly with the transpiration, so that the causative factors of reciprocal 

 leaf structures themselves appear to be reciprocal. The mechanics of the process, 

 that is, the exact method whereby the reduction of transpiration influences tissues 

 so as to produce large air spaces, is for the present scarcely to be conjectured. 



The role of air spaces. — Air reservoirs. — Air spaces are of vital im- 

 portance in furnishing ready ingress and egress for oxygen and carbon 

 dioxid to and from the active cells of the leaf chlorenchyma in connec- 

 tion with respiration and carbohydrate synthesis; these spaces also 

 greatly facilitate transpiration, the significance of which is to be con- 

 sidered elsewhere. In ordinary lakes and ponds, oxygen and carbon 

 dioxid are comparatively abundant, hence the large air spaces there 

 appear to be without advantage, so far as aeration is concerned, unless 

 the absence of stomata makes the entrance and the exit of gases relatively 

 slow. In stagnant swamps and undrained ponds, however, the oxygen 

 supply often is scant; indeed, it is commonly believed that the lack of 

 oxygen accounts for the quick decay of algae when transferred from 

 running streams to standing water. In such habitats, then, capacious 

 air spaces may be of much value as oxygen reservoirs. Green plants in 

 their synthetic processes give off much more oxygen than is utilized 

 in respiration, so that the presence of large air chambers permits the 

 accumulation rather than the complete dispersion of this oxygen. It 

 has been shown that in the water lilies the maximum oxygen content 

 of these air chambers is at sunset, at the close of a day of synthetic 

 activity, whereas the maximum for carbon dioxid occurs at sunrise, by 

 reason of the accumulation of the products of nocturnal respiration. 

 The carbon dioxid utilized in synthesis probably is more abundant in 

 the average waters than in the air, so that air reservoirs are of doubtful 

 efficacy in connection with that process. Furthermore, in aerenchyma 

 and in other lacunar tissues in many hydrophytes chlorophyll is absent. 



Buoyancy. — In most water plants air chambers serve to give buoyancy 

 to the various organs. The position assumed in the water by floating 



