BY SELF-ADAPTATION TO THE ENVIRONMENT. 233 
M. de Candolle has also called attention to the advantages of 
long roots in enabling the plants to resist extremes of tempera- 
ture. He says:—“ L’action de la température est trés-sensible 
a la surface des sols, et l’est moins & une certaine profondeur ; 
d’ou il résulte, 1°, que, dans un terrain donné, les plantes a 
racines profondes résistent mieux aux extrémes de la tem- 
pérature que celles 4 racines superficielles; 2°, qu’une plante 
donnée résiste mieux aux extrémes de la température dans un 
terrain plus compacte, ou moins bon conducteur du calorique, ou 
moins doué de la faculté rayonnante, que aans un sol ou trop 
léger ou bon conducteur, ou rayonnant fortement le calorique ; 
3°, la nature des plantes et celle du sol étant donneés, les 
plantes résistent mieux au froid dans une atmosphere séche, et 4 
la chaleur dans une atmosphére humide” *. 
The cause of the long tap-roots of so many desert plants is 
the well-known responsive power of the apices to moisture, or 
hydrotropism. Similar phenomena may not infrequently be seen 
in England. Thus if, for example, the tip of the root of a seedling 
turnip gets into a field drain-pipe, it may grow to a length of 
some yards, of course never producing the turnip ft. As water is 
often to be found at various depths below the surface of the 
desert, the roots stimulated by ascending moisture continue to 
grow downwards till they attain very great lengths. 
Duration.—Some desert plants are usually annuals, the 
majority being perennials. A feature, bowever, which Dr. 
Volkens notices is that these characters are particularly liable to 
change in desert plants according to circumstances. Plants 
which normally live but one year, as species of Savignya, Poly- 
carpon, Malva, Trigonella, Ifloga, &c., may survive two or more 
years ; while perennials, like Capparis, Tamaria, Nitraria, Retama, 
Acacia, &e., may become annuals. The fact is that it simply 
depends upon the depth to which the primary tap-root descends 
so as to secure a more continuous supply of water below the 
surface, which enables the plant to survive the hot season. 
Mr. Scott Elliot has observed the same fact in South Africa f. 
In speaking of the prevalence of the “ericaceous” type, including 
350 species of Erica itself, he remarks :—“It is not, I think, 
* «Essai élémentaire de Géographie Botanique,’ Dict. Sci. Nat. tom. xviii. 
t+ I have such a specimen, which was brought to the late Prof. J. 8. Henslow 
by a villager in Suffolk about the year 1890. 
t Op. cit., Trans, Bot. Soc. Ed. 1891, p. 244 [supra, p. 228). 
LINN. JOURN.— BOTANY, VOL. XXX. 8 
