the Embryos and Seedlings of the Cactaceae. 435 
E. ingens , which is normally very red, no colour at all appeared 
when I placed caps of tinfoil over the embryos as soon as germi- 
nated. The appearance of the colour, therefore, is a clear case 
of irritable response to the action of light as a stimulus. But 
there is another distribution of colour which is noticeable. In 
Echinoc actus Wislizeni , Opuntia Engelmanni occidentalism and 
some other species, the colour is not general, but is limited 
to a single bright red spot on the outside of the base of 
the cotyledons ; and moreover the leaves of the epicotyl 
(particularly in Cereus grandiflorus ) show the same feature. 
I think this is the result of the appearance of the colour only 
at the time when the leaves are very young and still folded 
with their faces against one another and their backs only 
exposed to the light. But it is not confined to the leaves, 
for the colour in some species only runs in streaks in the 
depressions between the ribs of the stem 1 . 
In seeking an explanation of the presence of the red colour, 
we must recall the fact already stated that it appears only in 
response to light as a stimulus ; and it is most intense in the 
forms which live exposed to the greatest brightness. It does 
not appear at all in Pereskia aculeata , the climbing species 
of Cereus, Phyllocactus , R hip satis, and those Platopuntiae 
growing in the less extreme deserts, such as Opuntia vulgaris 
and Rafinesquii , and it is best developed in the most extreme 
desert-forms of Mamillaria , Anhalonium , and Echinoc actus. 
In other words, the more mesophytic the habitat, the less 
the colour; the more xerophytic, the more the colour. There 
are two explanations of similar red colour elsewhere. First, 
according to Stahl, it may serve to absorb some of the light- 
rays, and convert them into heat which is of use in the 
processes of growth at a time when there is none too much 
heat available. Secondly, it may serve as a light-screen, 
cutting off the rays of the violet end of the spectrum, which 
act injuriously upon the living tissues. On the whole, in 
1 The spot of colour at the base of the leaf is not uncommon elsewhere. I have 
noticed it on the base of the petiole in Lysimachia alnifolia , in a species of Maple, 
and elsewhere ; also in Salicornia herbacea. 
