Again, the peculiar nocturnal movement of the left-hand coty-
[page 316]
ledon of Trifolium strictum, in combination with that of the first true
leaf. Lastly, the wide distribution in the dicotyledonous series of plants
with cotyledons which sleep. Reflecting on these several facts, our
conclusion seems justified, that the nyctitropic movements of cotyledons,
by which the blade is made to stand either vertically or almost vertically
upwards or downwards at night, has been acquired, at least in most cases,
for some special purpose; nor can we doubt that this purpose is the
protection of the upper surface of the blade, and perhaps of the central
bud or plumule, from radiation at night.
[page 317]
CHAPTER VII.
MODIFIED CIRCUMNUTATION: NYCTITROPIC OR SLEEP MOVEMENTS OF LEAVES.
Conditions necessary for these movements--List of Genera and Families,
which include sleeping plants--Description of the movements in the several
Genera--Oxalis: leaflets folded at night--Averrhoa: rapid movements of the
leaflets--Porlieria: leaflets close when plant kept very dry--Tropaeolum:
leaves do not sleep unless well illuminated during day--Lupinus: various
modes of sleeping--Melilotus: singular movements of terminal leaflet--
Trifolium--Desmodium: rudimentary lateral leaflets, movements of, not
developed on young plants, state of their pulvini--Cassia: complex
movements of the leaflets--Bauhinia: leaves folded at night--Mimosa pudica:
compounded movements of leaves, effect of darkness--Mimosa albida, reduced
leaflets of--Schrankia: downward movement of the pinnae--Marsilea: the only
cryptogam known to sleep--Concluding remarks and summary--Nyctitropism
consists of modified circumnutation, regulated by the alternations of light
and darkness--Shape of first true leaves.
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