USES OF SLEEP-MOVEMENTS, ETC. 643 



&c., when in the direct rays of the sun. Still it is of course doubtful whether in 

 such cases the effect is to be ascribed to the light or to the heat. 



If in conclusion we raise the question as to the purpose and uses for the 

 plants concerned of all the periodic movements and changes directly resulting from 

 stimuli here described, of course only a special consideration of the mode of life 

 of each individual species could afford the answer in detail ; still their utility can 

 be recognised in a general sense even without that. That flowers with rare 

 exceptions open in the morning, as the brightness and warmth increase, and close 

 in the evening, is evidently connected with the function of pollination — i. e. with the 

 transference of the pollen from one flower to the female organ of another flower 

 of the same species. For this transference is effected by insects, which as a rule only 

 visit the flowers in bright and warm weather : at night, when the conditions are 

 different, the sexual organs of the flower are protected by the closure of the 

 corolla against excessive cooling by radiation and wetting with dew, and probably 

 from several other dangers. The opening and closing of foliage-leaves may in some 

 cases enhance the above described protection of the sexual organs, though as a rule 

 it may be assumed that by pronounced erection or depression of the laminae of 

 foliage-leaves in the evening, the excessive cooling of the highly important 

 chlorophyll-tissue during the night is avoided ; on knowing that these thin tissue- 

 lamellae may be cooled by mere radiation at night, particularly when the sky is clear 

 and serene, to 5-8" C. below the temperature of the surrounding air, it is obvious 

 that even during nights when the air is at 5°-6° C. such a cooling may fall below 

 the zero point and lead to the danger of freezing. The extended diurnal position 

 of the foliage-leaves is simply and only adapted for temperatures and lights favour- 

 able to vegetation, and only useful for assimilation within these limits ; wherever 

 the conditions are unfavourable in this respect, particularly in the case of thin 

 delicate foliage-leaves, they become closed or assume the profile position, as well in 

 excessive sun-light as in the darkness and cold of the night. 



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