GRAVITATION. 125 



If an herbaceous plant as the Satoimo be frozen and be 

 quickly thawed, its vitality will be generally destroyed. 



From the preceding experiments it will be seen that 

 common plants can live only within certain limits of tem- 

 perature. From several experiments it has been assumed 

 that the vital processes of common plants begin at a certain 

 number of degrees above freezing-point and cease at about 

 50° C. ; and that the most favourable conditions of vital 

 activity are generally attained at temperatures from about 

 25° to 30° C. 



2. LIGHT. 



If the stems of the Udo (Aralia cor data) be covered 

 with soil, they will become whitish after several days. 



If the seeds of the Daidzu be germinated in complete 

 darkness, the young plants produced will assume a whitish 

 colour. 



If the same seeds be germinated in places fully exposed 

 to light, the young plants produced will assume a green 

 colour. 



From the preceding experiments it is clear that the 

 formation of chlorophyll is dependent upon light. 



The formation of starch in chlorophyll corpuscles, as has 

 been seen, is also dependent upon light.. 



Light is therefore indispensable to green plants. 



In regard to growth, many organs are considerably 

 retarded by light ; the stems of many plants are caused by 

 this influence to decline towards the sun. 



3. GRAVITATION. 



If a young plant as the Daidzu planted in a flower-pot be 

 inverted, the apex of the stem will be gradually directed 

 upward and the apex of the root downward. 



