THE MANIFESTATIONS OF LIFE 57 



earth out of doors, when a sturdy stalk with abundant 

 dark green leaves assumes a vertical growth. 



Whoever has beautified his window with growing 

 plants must have observed how regularly the leaves turn 

 toward the light, and how necessary it is to turn the pot 

 around day after day if symmetrical development of the 

 plant is desired. The movement of the leaves depends 

 upon positive heliotropic movements of the stems and 

 petioles by which the leaves are kept at right angles to 

 the incident light, thus exposing the entire upper surface, 

 and enabling its superficial cells to benefit by its influence. 

 The study of the entire plant usually shows that the root 

 turns away from the source of light as the leaves turn 

 toward it. The leaves are therefore positively, the 

 roots negatively, heliotropic. 



Certain plants such as Mimosa partially or completely 

 close the leaves when the sunlight wanes, to open them 

 again when it waxes. Many flowers close during the 

 night to open again when the morning sun strikes them. 

 The morning-glory and dandelion are familiar examples. 



It is, of course, difficult to exclude the heat accompany- 

 ing the sun's rays as a factor in these movements, yet 

 the amount of heat in the feeble light of the cellars in 

 which potatoes sprout can scarcely be accompanied by 

 sufficient heat to explain the behavior already pointed 

 out, and the disastrous effects of absence of light in the 

 presence of heat would indicate that it is the light and 

 not the heat that effects the reaction. Thus, if a well- 

 grown, healthy plant be transferred to a warm but dark 

 room, in spite of the careful maintenance of all other 

 healthy conditions, the plant soon sickens and the leaves 

 and flowers fall off. 



The positive heliotropic reactions subserve a useful 

 purpose in bringing the essential organs of the plant into 

 the sunlight without which its important functions are 

 impossible. Thus, in the dark no chlorophyl can be 

 formed, and without chlorophyl the proper nutrition, 

 growth, and perfection of the higher plants cannot be 

 carried on. 



