438 The American Naturalist. [May, 
“II. The XEROPHYTE-VEGETATION is the opposite extreme, whose 
plants grow upon stony soil, or, at least, during a greater portion of the 
year, in soil scarcely supplied with water, and in dry air. The water 
content can, indeed, if it is at a minimum amount, be less than 10 per 
“III. The HALOPHYTE-VEGETATION is closely related morphologi- 
cally to the foregoing, but merits a separate consideration, an opinion 
that is confirmed, among others, by the investigations of Stahl. It isa 
very extreme vegetation, that is limited to salty soil and whose morpho- 
logical peculiarities appear likewise to be caused by the regulation of 
evaporation. 
“TV. The MESOPHYTE-VEGETATION includes the communities that 
are adopted to a soil and air of medium moisture, and to a soil also 
which is not particularly salty. The plants form a morphological and 
anatomical standpoint, are not especially extreme in their characters.” 
Space will not allow even an outline of the interesting chapters fol- 
lowing, but for illustration we will glance at the xerophytes. 
In the xerophytes adaptation has taken place along two lines, reduc- 
tion of the transpiration during the critical period, and a development 
of especial means for gathering or storing water. Regulation of tran- 
spiration may be accomplished by reducing the evaporation in the 
following ways: 1. Periodical reduction of evaporating surface; de- 
ciduous trees dispense with their leaves in winter, bulbous plants 
relinquish the exposed parts in the dry season, annual plants pass this 
season in the seed stage ; the leaves, especially of grasses or the thallus, 
may roll up in various ways. 2. The leaves change their position so as 
to regulate the amount of light, and consequently the amount of heat 
which they receive from the sun; many Leguminose place their leaflets 
in a vertical position during the heat of the day (para-heliotropism). 
Even our much-despised purslane shows this on a hot day ; the common 
impression being that it is wilting—it knows better than that. 3. A 
permanent vertical position of the foliage organs. If the leaves arè 
upright they tend to throw their surface into a meridional plane (com- 
pass plants). The leaves may be directed outwardly and twisted on 
their petioles (Eucalyptus), or hang on slender petioles (cotton-wood). 
The petioles may be laterally flattened and take the function of foliage 
(many Acacia). 4. The surface may be reduced in proportion to the 
volume, such as the needle-like leaves of the pine, the succulent leaves 
of the sedums, or where the stem acts as foliage and the leaves are re- 
duced to scales, as in the cactus or asparagus. 5. The evaporation mey 
be hindered by a coating of hairs, a common contrivance in dry region 
