162 



BOTANY: PRINCIPLES AND PROBLEMS 



of gravity is known as a geotropism (Fig. 80). Stems ordinarily 

 tend to grow in a direction opposite to the force of gravity and are 

 thus negatively geotropic. Primary roots and certain other 

 portions of the root system, on the contrary, tend to grow directly 

 toward the center of the earth and are thus positively geotropic. 



Fig. 80. — Geotropism. Four kernels of corn which have germinated in 

 different positions. The young root in every case has grown downward and the 

 young shoot upward. 



Most leaves tend to take up their position at right angles to the 

 force of gravity and many lateral roots also grow in an approxi- 

 mately horizontal direction. Such organs are said to be diageo- 

 tropic. The advantage of these specific tropisms to the plant is 

 obvious. 



Plants differ considerably in their inherited adaptations to the 

 influence of gravity. Stems of prostrate plants have lost their 

 negative geotropism and may even develop into rootstocks or 

 tubers which react toward gravity like roots. The responses of 

 flowers and fruits to this stimulus are also very diverse and for 

 the most part seem to be advantageous to the plant. 



The mechanism of stimulus and response has been more care- 

 fully studied in the case of geotropism than with other environ- 

 mental factors. If very young seedlings in which the' root and 

 stem are just appearing are fixed in any position whatever, the 

 young root will invariably grow downward and the young stem 

 upward (Fig. 80). By attaching such seedlings to a disc and 

 revolving it rapidly, centrifugal force may be developed which is 

 greater than gravity, and in such a case the young plants orient 



