MOVEMENT 371 



trolytic current), has little significance in nature. If the ions are 

 responsible for the plant movements seen in an electrolytic solution, 

 then galvanotropism may also be a form of chemotropism. 



Of more importance is thigmotropism or haptotropism, which is 

 irregular growth produced by contact, friction, etc. This is of 

 especial importance in twining plants and those with tendrils. 

 The side in contact with the support grows less than the opposite 

 side, and twining results. Tendrils often have tactile spots which 

 are exceptionally sensitive. The initial bending may be produced 

 by a change in turgor, which is followed by differences in growth. 

 The twining is thus a result of autonomic nutations, of turgor 

 changes, and of permanent growth changes, stimulated by the 

 friction of the support. Since light and gravity also play a part, 

 it is seen that the twining of stems and tendrils is a very com- 

 plicated phenomenon. 



Traumatropism, or the differential growth due to wounding, 

 has already been discussed (Chap. XXVII). These many citations 

 to other chapters should serve to impress upon the student the 

 fact that the phenomena of living matter are closely interrelated. 

 It is not possible to discuss growth without considering movement 

 and irritability or, vice versa, to consider movement apart from 

 growth. All of these coordinated and related processes make up 

 the phases of the unexplained mystery called life. 



QUESTIONS 



1. Why are plants less motile than animals? 



2. What are coenocytes, fern prothallia, rhizomes, flagella? 



3. Describe the movement of Oscillatoria. 



4. Distinguish between phototaxy, photonasty, and phototropism. 



5. What are floral clocks? 



6. What is the derivation of the words thigmotaxy, autonomic, paratonic, 

 traumatropism, and rheotropism? 



7. Give two reasons why leaves in the shade are generally greener than 

 those in the bright sunlight. 



8. "The presentation time multiplied by the square root of the wave fre- 

 quency is nearly a constant." Explain clearly. 



9. Guttenberg (1924) found that when Avena and Coleus were illuminated 

 by oblique light, the response was proportional to the sine of the angle made by 

 the beam with the plane of the illuminated surface. Is this in agreement with 

 the results discussed in the previous chapter in connection with geotropism 

 and the laws of acting stimuli? 



10. Which is the better word for the movements of oxalis leaflets — nycti- 

 tropic or photeolic? Why? 



