Indeed, if we put aside the radiate
and the color-varieties of flowers and foliage, hardly any cases can be
cited. We have dealt with this question in a former lecture, and may now
limit ourselves to the positive color-varieties.
The latency of the faculty of producing the red pigment in leaves must
obviously be accepted for nearly the whole vegetable kingdom. Oaks and
elms, the beautiful climbing species of Ampelopsis, many conifers, as
for instance _Cryptomeria japonica_, some brambles, the Guelder-rose
(_Viburnum Opulus_) and many other trees and shrubs assume a more or
less bright red color in the fall. During summer this tendency must have
been dormant, and that this is so, is shown by the young leaves of oaks
and others, which, when unfolding in the spring show a similar but paler
hue. Moreover, there is a way of awakening the concealed powers at any
time. We have only to inflict small wounds on the leaves, or to cut
through the nerves or to injure them by a slight bruising, and the
leaves frequently respond with an intense reddening of the living
tissues around and especially above the wounds. _Azolla caroliniana_, a
minute mosslike floating plant allied to the ferns, responds to light
and cold with a reddish tinge, and to shade or warmth with a pure green.
The foliage [240] of many other plants behaves likewise, as also do
apples and peaches on the insolated sides of the fruits.
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