CH. XIV] THE HELOPHYTES 571 



chlorenchyma in vertical ribs instead of leaves, which are 

 wanting; typified by Cacti and some Euphorbias. This 

 is the extreme xerophytic type. 



D. THE UNDRAINED HABITATS, AND THE VEGETATION 

 FORMS CALLED HELOPHYTES. 



In the habitats where water stands constantly, viz. in 

 swamps, bogs, marshes, and along river and lake margins, 

 the relatively massive roots and rootstocks of Spermatophytes 

 can obtain from solution in water only a small part of the 

 oxygen needed in their respiration, especially as the supply 

 is renewed slowly where its diffusion is impeded by presence 

 of the soil. Hence the oxygen for the under-water parts 

 must be supplied from other sources. To some extent it 

 may be derived from anaerobic or " intracellular " respiration 

 (page 171), but chiefly it is conducted from the atmosphere 

 through the aerial parts. Correspondingly, plants which 

 grow in such places possess greatly enlarged air passages in 

 stemsor_jDetial^ whereby is explain5r~Ehe soft-spongy 

 tSxTure of true marsh and bog plants. Superficially these 

 plants, especially if floating, are often confounded with hydro- 

 phytes ; but in fact these marsh plants or helophytes are true 

 aerophytes, since they take their supply of carbon dioxide 

 and oxygen, even for their immersed parts, through inter- 

 cellular spaces from the atmosphere. Every gradation 

 exists between soil saturated with water and standing water 

 without soil, but physiologically they are practically identi- 

 cal. The principal helophytic vegetation forms are the 

 following. 



SWAMP FLAGS; vertical sword-shaped leaves rising from 

 submerged rootstocks aerated through passsages from the 

 leaves ; typified by Cat-tails, true Flags (Acorus), Iris, with 

 Saw Grass of the Florida Everglades, and many of tropical 

 marshes. MARSH REEDS ; vertical, slender, cylindrical shoots 

 from submerged rootstocks, aerated through specially spongy 

 pith; typified by Bulrushes, some Sedges, and Horsetails. 



