THE MANIFESTATIONS OF LIFE 55 



isms; some, like the red and yellow rays, being useful; 

 others, like the blue and violet rays, being prejudicial in 

 action. 



Many living organisms flourish under the direct rays 

 of a tropical sun, others are quickly killed by direct sun- 

 light, still others seem to find the most favorable condi- 

 tions in the perpetual darkness of caverns or the depths 



FIG. 10. A seedling of the White Mustard in a water culture which has first 

 been illuminated from all sides and then from one side only. The stem ia 

 turned toward the light, the root away from it, while the leaf-blades are ex- 

 panded at right angles to the incident light. K,K, Sheet of cork to which the 

 seedling is attached. (Strasburger, Noll, Schenck, and Karsten.) 



of the sea. It is thus possible to describe positive and 

 negative heliotropism among both plants and animals. 



Certain bacteria are highly susceptible to light, and 

 some species are quickly killed when exposed to the 

 direct rays of the sun. Bacteria and indeed most fungi 

 seem to flourish best in diffused light; a few appear to 

 grow best in the dark, and sometimes certain functions 



