This fact leads to the suspicion that
D. gyrans is descended through a unifoliate form (of which some exist) from
a trifoliate species; and that the little lateral leaflets reappear through
reversion. However this may be,
[page 417]
the interesting fact of the pulvini or organs of movement of these little
leaflets, not having been reduced nearly so much as their blades--taking
the large terminal leaflet as the standard of comparison--gives us probably
the proximate cause of their extraordinary power of gyration.
[page 418]
CHAPTER VIII.
MODIFIED CIRCUMNUTATION: MOVEMENTS EXCITED BY LIGHT.
Distinction between heliotropism and the effects of light on the
periodicity of the movements of leaves--Heliotropic movements of Beta,
Solanum, Zea, and Avena--Heliotropic movements towards an obscure light in
Apios, Brassica, Phalaris, Tropaeolum, and Cassia--Apheliotropic movements
of tendrils of Bignonia--Of flower-peduncles of Cyclamen--Burying of the
pods--Heliotropism and apheliotropism modified forms of circumnutation--
Steps by which one movement is converted into the other--
Transversal-heliotropismus or diaheliotropism influenced by epinasty, the
weight of the part and apogeotropism--Apogeotropism overcome during the
middle of the day by diaheliotropism--Effects of the weight of the blades
of cotyledons--So called diurnal sleep--Chlorophyll injured by intense
light--Movements to avoid intense light
SACHS first clearly pointed out the important difference between the action
of light in modifying the periodic movements of leaves, and in causing them
to bend towards its source.
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