564 A TEXTBOOK OF BOTANY [Ch. XIV 



buoyantly towards light ; typified by Sea Lettuce (Fig. 291), 

 and especially by the "Sea Moss" Red Algae (Fig. 310). 

 Sedentary Waterweeds ; Spermatophytes, with true leaves 

 and stems, attached by true roots, but slender-elongate, 

 thin or cut like Algae, rising buoyantly towards the light, 

 several feet long in quiet places, with flowers pollinated 

 above, on, or under the surface ; typified by the Pondweeds 

 (Fig. 372), Eelgrasses (Fig. 196), Bladderworts (Fig. 399), 

 and others. Intertidal Rockweeds; thalloid, divided, 

 attached by firm holdfasts, and tough-elastic, yielding to 

 wave action, and relatively massive ; typified by the Rock- 

 weeds of the coast (Figs. 132, 308). Allied is the form of the 

 Kelps, with fronds floating at times, buoyed on long stalks 

 (Figs. 133, 306-7). 



(2.) The Aerial Habitats, and their Vegetation Forms, 

 called a'erophytes. 



The fundamental distinction between water and land 

 habitats consists in the fact that water plants absorb their 

 carbon dioxide and oxygen from solution in the water, 

 while land plants absorb them direct from the air. The condi- 

 tions of absorption of water and mineral salts, and the rela- 

 tions to light, are substantially the same in both. Hence 

 the contrast between water and land habitats is not between 

 water and land (i.e. soil), but between water and air; and 

 therefore we call land plants not geophytes, but aerophytes 

 (page 561). 



The physical conditions to which the structure of aero- 

 phytes must conform, are well known. On land the primal 

 necessities of plant life do not exist intercommingled, as in 

 water, but from the point of view of plant life occur in two 

 strata, viz., the light and carbon dioxide in the atmosphere 

 above, and the water and mineral salts in the soil below, 

 oxygen being present in both strata where the soil is well 

 aerated. Correspondingly, aerophytes are differentiated 

 into two primary parts, green shoot and absorbing root, 



