BY SELF-ADAPTATION TO THE ENVIRONMENT, 223 
cold as well as extreme dry heat; as there will be a greater 
number of cells in the layers of cork filled with air, thus making 
bad conductors between the external air and the cambium layer 
and alburnum. Thus old trees resist cold better than young 
ones. On the other hand, endogenous trees having no bark 
only grow in warm climates. In some, as the Date, the bases 
of the leaves, especially if they decompose into a hairy covering, 
may supply the place of it. 
The evidence in support of the assertion that spines are the 
direct outcome of the environment also rests upon the well- 
known fact that there are many instances of plants losing their 
spines altogether when grown under other circumstances. This 
variableness in the spinescent character of plants is no new ob- 
servation. Thus, G. G. Kiichelbecker, in a ‘ Dissertatio botanico- 
physica de spinis plantarum’ (a.D. 1756), wrote as follows :— 
“Sunt autem quaedam plantae quae eundem semper et vbique 
seruant in extensione superficiei habitum, cum contra ea aliae, 
pro varia soli et culturae indole, formam hance alias sibi propriam 
deponant, vel tamen maximam sui partem mutent, ita vt, quae 
glabrae antea erant, nunc inaequalitatem magis minusue emi- 
nentem induant, atque suum plane deposuisse videantur habitum 
superficiei pristinum ” (pp. 9, 10). He then refers to Linneus’s 
Philos. Bot. p. 215, § 272 :—‘‘Spinosae arbores cultura saepius 
deponunt spinas in hortis...... Hirsuties loco et aetate 
facillime deponitur.” Again, 1. ¢. p. 247, § 316 :—‘ Solum 
mutat plantas, vnde varietates enascuntur, et, mutato eodem, 
redeunt. Hine Acanthi molles et aculeati: Cinarae aculeatae 
et non aculeatae.”’ 
Similarly, at the present day Pears, species of Rose and of 
Prunus, &c., are well known to lose their spines under cultivation. 
Ononis spinosa, L., has an excessively spiny variety, horrida, grow- 
ing in maritime sands. It is much less spiny in waste places by 
roadsides, &c., and becomes the variety inermis elsewhere. This 
latter form of the “subspecies” repens can be produced, tem-- 
porarily at least, at will; for when the ordinary spiny form 
O. spinosa is grown either in avery rich soil or with an abundance 
of water and a moist atmosphere, whether the plants be raised 
from cuttings or seeds, they gradually lose their spines; those 
first formed under these conditions are much reduced in size and 
in rigidity. Hereditary influence is too strong to arrest them 
at first altogether; although none are produced later on vigorous 
