BY SELF-ADAPTATION TO THE ENVIRONMENT. 239 
probability absorb dew, as well as protect the surfaces against a 
loss of water. The marginal inrolling of the blade is, of course, 
an additional protection. To such an extent does this occur, 
that many blades of desert-grasses are perfectly cylindrical; the 
upper surface, which is the especially grooved one, being entirely 
concealed from view, as may be well seen in Volkens’s figure of 
Aristida ciliata*. In some cases, as in this grass, certain hairs 
assume a papillate form immediately over the stomata. 
As another illustration which seems to support M. Mer’s con- 
tention that hairs are, ceteris paribus, a result of compensation, 
M. Lesage +t found in a root of the second order of Phaseolus, 
which was much longer than the primary root, that the portion 
outside the water was covered with numerous root-hairs ; zear the 
water these hairs were elongated, while in the water they were 
much shorter, and finally disappeared altogether. In a transverse 
section it was seen that the cortical layers in the air contained 
smaller elements than those in the water; and in the central 
cylinder the xylem was proportionally more lignified in the aerial 
portion. 
The root of the bean was made the subject of similar obser~ 
vations. It was found that when numerous secondary roots were 
suppressed, the primary root was covered with numerous absorbing 
hairs. 
The above interpretation will, therefore, satisfacterily explain 
the existence of the hairiness of plants in the deserts ; for drought, 
aided by the barrenness of the soil, tends to arrest the develop- 
ment of parenchymatous tissues; and in proportion as this arrest 
is excessive, so is the compensating process of the production of 
hairs. Hence, just as with plenty of water ora good soil, as 
obtains under cultivation, plants tend to become less hairy than 
in the wild state, as e.g. the parsnip, 80, conversely, under aridity 
and a poor soil, hairiness becomes a characteristic and hereditary 
feature. 
Here again, therefore, if the above explanation of M. Mer’s 
be true, the very conditions which bring about the production of 
an excessive clothing of hair are precisely those against the 
severity of which this dense clothing is one of the very best of 
protections. _ 
The above interpretation receives indirectly an additional 
* Op. cit. pl. xvii. fig. 4 [supra, p. 220]. 
t Comptes Rendus, exii. 1891, p. 109. 
