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 Graminacez 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 accordingly. The halo- 
phytic strand-plants Psamma arenaria, Elymus arena- 
rius, Agropyrum junceum, and other Dune-species, as well 
as species of Azra, Festuca, Anthozanthuwm, Stipa, Lygeum, 
Aristida, &c. are examples. The heath-grasses—e.g. 
Festuca ovina, Nardus stricta, Molinia cerulea—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 
