This view that the sleep of leaves saves them from being chilled at night
by radiation, would no doubt have occurred to Linnaeus, had the principle
of radiation been then discovered; for he suggests in many parts of his
'Somnus Plantarum' that the position of the leaves at night protects the
young stems and buds, and often the young inflorescence, against cold
winds. We are far from doubting that an additional advantage may be thus
gained; and we have observed with several plants, for instance, Desmodium
gyrans, that whilst the blade of the leaf sinks vertically down at night,
the petiole rises, so that the blade has to move through a greater angle in
order to assume its vertical position than would otherwise have been
necessary; but with the result that all the leaves on the same plant are
crowded together as if for mutual protection.
We doubted at first whether radiation would affect in any important manner
objects so thin as are many cotyledons and leaves, and more especially
affect differently their upper and lower surfaces; for although the
temperature of their upper surfaces would undoubtedly fall when freely
exposed to a clear sky, yet we thought that they would so quickly acquire
by conduction the temperature of the surrounding air, that it could hardly
make any sensible difference to them, whether they stood horizontally and
radiated into the open sky, or vertically and radiated chiefly in a lateral
direction towards neighbouring plants and other objects.
Pages:
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475