76 



PEACTICAL BOTANY 



68. Air storage in stems and leaves. In many marsh and 

 water plants very extensive supplies of air are stored in the 



interior of the roots, rootstocks, 

 the ordinary stems, and the 

 leaves. This stored air consti- 

 tutes what has been well called 

 an mner atmosphere, by means 

 of which the respiration of the 

 plant is much aided, especially 

 at times when the whole plant 

 body is temporarily submerged. 

 In those marsh or water plants 

 which have the most extensively 

 developed air passages and cav- 

 ities they form a complex system 

 which extends all the way from 

 the stomata to the tips of the 

 roots. Often a large part of the 

 bulk of the plant body is occu- 

 pied l)y such air cavities, sur- 

 rounded by slight walls of solid 

 material. In the leaves of Pistia, 

 a iioating aquatic belonging to 

 the Arum family, 71 per cent of 

 the volume is occupied liy air 

 spaces, while m ordinary land 

 plants these spaces may occupy 

 less than 7 per cent of the total 

 volume of the leaf. 



An important mechanical use 

 is often subserved by stems or 

 leaves inflated with air, in buoy- 

 ing up the plant, as is well shown 

 by the duckweeds, the \'\-ater 

 liyacinth ( I'/n'Mnrnici), and the ^vat^■r chestnut (^Tnijui). j\Iany 

 seaweeds, as the rockwced, are thus liuoyed up. 



Fig. 62. A century plant, nearly 

 ready to blossom 



The flower stalk considerably devel- 

 oped and the outer leaves beginning 

 to shrivel and droop from loss of 

 food transferred to the flower stalk. 

 Photograph by G. D. Fuller 



