552 



ECOLOGY 



the presence of sunlight. Transpiration, involving the emission of 

 water vapor (evaporation), occurs in all aerial organs, being absent in 

 the water and slight in the ground or in very humid air. Transpiration 

 involves much the greatest gas movement, respiration much the least. 

 In the lowest plants, where each cell is in contact 

 vnth the surrounding medium, there are few or no air 

 cavities. In the higher plants, however, there are 

 systems of connecting intercellular air chambers, 

 communicating with the outside air by open- 

 ings (such as stomata) or by loose external tissues 

 (such as lenticels). Plants, therefore, have an 

 internal atmosphere, differing somewhat from 

 that outside, though tending to approach it by 

 diffusion. 



The structural features and variations of air 

 spaces. — The lower leaf chlorenchyma, the spongy 

 tissue, is especially rich in air spaces or lacunae (figs. 

 760, 761, 820), and a rather prominent air cavity 

 underlies each stoma. There are small intercellular 

 spaces between all of the chlorenchyma cells, those 

 between the palisade cells being narrow and in- 

 conspicuous, except as seen in cross section (fig. 

 790). The thin cell walls adjoining the air spaces 

 are of cellulose. Air chambers, often beautifully 

 symmetrical in arrangement (as in the stem of 

 Myriopkyllum, fig. 791), are particularly con- 

 spicuous in hydrophytes, where they occur in 

 all the vegetative organs, their total volume often 

 being greater than that of the cells. In submersed 

 leaves the entire mesophyll consists of spongy 

 parenchyma with large lacunae (fig. 1018), while 

 in floating leaves there may be a striking contrast 

 between the emersed palisade layer and the submersed sponge (fig. 

 805). Sometimes, as in the leaf of J uncus (fig. 792) and in the stem 

 of Zizania, large air chambers are partitioned off by diaphragms, 

 which often appear to the naked eye as cell walls. Air chambers are 

 said to be less abundant in plants of swift streams than in those of 

 ponds and swamps. Xerophytes, particularly succulent species, are 

 characterized by small and inconspicuous air spaces (figs. 835, 926). 



Fig. 792. — A part 

 of a leaf of Juncus 

 nodosus with a portion 

 cut away, disclosing 

 capacious air cham- 

 bers, separated from 

 each other by hori- 

 zontal plates, the dia- 

 phragms (d). 



