118 BIOLOGY. [Book ii. 



cause of life upon the surface of the globe. "With these reser- 

 vations, we cannot deny that solar irradiation is fixed, accumulates 

 in the plant, and that, in the infinitely numerous cases in which 

 the plant serves as animal alimenta;tion, this irradiation is 

 jireasured up by it, transforms itself into various series of 

 molecular vibrations, into heat, movement,, thought, etc. 



We have not to return here to the chief office fulfilled by 

 light in the nutrition of plants ; but, besides the phenomena of 

 synthetic chemistry accomplished by chlorophyll, light also 

 produces in plants certain secondary phenomena, which probably 

 depend upon the chlorophyllian property. Thus many plants 

 sleep in the night, that is to say, droop their leaves or follicles 

 more or less, or rather gather them up along the stem or 

 principal petiole. This phenomenon assuredly depends upon 

 the light, since De Candolle was able to make sensitive plants 

 sleep in the day in artificial darkness, and, on the other hand, 

 succeeded in waking them at night by the light of lamps.' This 

 so-called slumber, this sinking down of the leaves, arises probably 

 from a diminution of the turgescence of the tissues, and this 

 diminution is assuredly the result of the inaction of the 

 chlorophyllian cells, which, no longer exhaling, no longer 

 forming organic syntheses, summon less water into the petiolary 

 canals, the meatus, and so on. 



If the luminous undulations are, as it were, the soul of vegetal 

 life, caloric undulations are not less indispensable to the nutrition 

 of plants. Above and below a certain temperature, vegetal life 

 ceases. Speaking generally, the thermometric limits of vegetal 

 life are degree and 50 degrees.'^ It must be noticed that the 

 cellular juices, being liquids very full of substances in solution, 

 do not congeal at degree. M. TJloth has seen seeds of Acer 

 platanoides and triticum germinate between the fragments of ice 



• Mlmoire sw V Influence de la LvmUre ariificielle sur Us Plantes (ifto, dis 

 Savants Strangers de VInstitvi, t. i. ) 

 ^ Sachs, Ueber die obere Temperatwrgrenze der Vegetation {Flora, 1864). 



