158 SCIENCE FROM AN EASY CHAIR 



other things. The food of all living things comes ulti- 

 mately from plants which, in the presence of sunlight, 

 and only in that presence, and in virtue of its action 

 upon their green leaves, manufacture starch and sugar 

 from the carbonic acid which exists in the air and water 

 around them, whilst they are also thus enabled to take 

 up nitrogen, and so to form their living substance or 

 protoplasm. At night those particles or cells of the 

 living protoplasm of plants which are furnished with 

 transparent green granules, so as to entangle the sun- 

 light, and by its aid feed on carbonic acid, cease this 

 work. They necessarily repose from their labour because 

 the light has gone. This is the simplest example of the 

 sleep of living things. And that here, too, as in higher 

 creatures, sleep is not a merely negative thing a mere 

 cessation is shown by the fact that it is at night that 

 other changes go on in the plant. The manufactured 

 food takes effect on the cells or particles nourished by it ; 

 in the night the well-fed, enlarged " cells " in the growing 

 parts of many plants slowly divide each one into two, 

 and each of these again into two, and so on, so as to 

 increase their total number and produce growth and 

 development of the plant. This alternation of activities 

 in day and night occurs even in the invisible microscopic 

 vegetation of pools and streams. Animals even the 

 most minute, only visible with a strong microscope 

 move about in search of " bits " of food in fact, bits of 

 other animals or of plants and they, too, are, with 

 special exceptions, checked in their search for food by 

 the darkness, for even extremely minute and simple 

 animals are guided in their search by light that is to 

 say, by a more or less efficient sense of sight. Thus we 

 see that in a general way the sun is truly the ruler of 

 life, and that when he is hidden from us we all become 

 quiescent, a condition which may be rightly considered 



