670 PLANT MOVEMENTS 



on one side of the pulvinus coupled with a maintenance or even an increase 

 in turgor in the cells on the opposite side of the organ. 



Discussion Questions 



1. Roots of poplar trees and many other woody species often cause trouble by 



plugging up sewer lines and drains with masses of profusely branched roots. 

 This has been cited as evidence of positive hydrotropism. Can you see 

 any objections to such an explanation? 



2. What explanations can you suggest to account lor the failure of most roots 



to exhibit negative phototropism? 



3. The influence of relative moisture content upon the direction of root growth 



is sometimes demonstrated by planting seeds in a tray with a cheesecloth 

 bottom. The tray is then filled with wet moss and suspended in the air 

 at an angle of 45° to the vertical. When the air around the tray is 

 saturated the roots grow straight down, but when the air is appreciably 

 below saturation the roots bend and grow parallel to the bottom of the 

 tray. Can the results of this experiment be interpreted in any other way 

 than as a reaction to moisture? 



4. Leaf mosaics are commonly interpreted as a direct result of light of different 



intensities. Are any other explanations possible? 



5. The very rapid movements of the leaf blades of the Venus Fly Trap are 



attributed to growth while the rapid movements of the leaves of the Sensi- 

 tive Plant are classed as turgor movements. How can growth movements 

 be distinguished from turgor movements? 



Suggested for Collateral Reading 



Benecke, W., and L. Jost. Pflnnzcnphysiologie, Vol. IL G. Fischer. Jena. 



1924. 

 Boysen Jensen, P. Growth hormones in plants. Translated and revised by 



G. S. Avery, Jr., and P. R, Burkholder. McGraw-Hill Book Co. 



New York. 1936. 

 Goldsmith, G. W., and A. L. Hafenrichter. Anthokinetics. Carnegie Inst. 



Wash. Publ. No. 420. Washington. 1932. 

 Pfeffer, W. The physiology of plants. 2nd Ed. Translated and edited by 



A. J. Ewart. Vol. IIL Clarendon Press. Oxford. 1906. 

 Stiles, W. An introduction to the principles of plant physiology. Methuen 



and Co. London. 1936. 

 Went, F. W., and K. V. Thimann. Phytohormones. Macmillan Co. New 



York. 1937. 



Selected Bibliography 



Avery, G. S., Jr. Differential distribution of a phytohormone in the develop- 

 ing leaf of Nicotiana and its relation to polarized growth. Bull. Torr. 

 Bot. Club 62: 313-330. 1935. 



Blackman, V. H., and S. G. Paine. Studies in the permeability of the pulvinus 

 of Mimosa pudica. Ann. Bot. 32: 69-85. 191 8. 



