36 XEROPHYTES AND HALOPHYTES [CH. 



rule, and even sand-loving grasses are not real excep- 

 tions. 



The physiognomy of the grasses has always been 

 regarded as a striking one, and Humboldt classed it as 

 one of his 19 types of vegetation. As is well known 

 they are sociable plants, often covering enormous areas 

 — prairies, alps, steppes, &c. — with a few species, alone or 

 densely scattered throughout a mixed herbage. They also 

 represent characteristically the sun-plants, the erect leaves 

 exposing their surfaces obliquely to the solar rays, and 

 being often folded and nearly always narrow. 



The dead remains of these sociable grasses are an 

 important factor in protecting the soil against drought 

 and in facilitating humification, as well as in covering up 

 plants during long winters or dry seasons, keeping the 

 ground warmer and moister, and generally lessening the 

 effect of extremes. 



Many Graminacese are pronounced xerophytes, the 

 epidermis often being developed as a water-storing 

 tissue, while the erect leaves roll themselves in intense 

 light, the stomata being situated accordiiigly. The halo- 

 phytic strand-plants Psamma arenaria, Elymus arena- 

 rius, Agropyrum junceum, and other Dune-species, as well 

 as species of Aira, Festuca, Anthoxanthwm, Stipa, Lygeum, 

 Aristida, &c. are examples. The heath-grasses — e.g. 

 Festuca ovina, Nardus stricta, Molinia ccerulea — also 

 come under this category. 



Many of the strand-plants (halophytes) Agropyrum, 

 Psamma, Elymus, are covered with waxy bloom, and have 

 long rhizomes which bind the sand and form new soil, a 



