INFLUENCE OF EXTERNAL CONDITIONS ON GROWTH 



3°3 



experiments may be mentioned as examples of this sort of study. A thread was 

 attached to the tip of the shoot to be experimented upon, was passed over a 

 pulley above, and bore a weight on its free end, the downward pull of the latter 

 being transmitted so as to produce an upward pull upon the end of the shoot. 

 The following table presents the results of some of Hegler's measurements of 

 the daily rates of elongation of various plants with and without traction and 

 with different amounts of traction. 



The first effect of applying an upward pull to the plant is seen to be a pro- 

 nounced retardation of growth, but the rate of elongation afterwards increases, if 

 the same traction is continually applied, so as to equal and finally even to exceed 

 the rate of the control plant without traction. Frequently, as with the sun- 

 flower seedlings and Dahlia shoots of the above table, the period of growth 

 retardation lasts only about one day, but in some cases, as with the hemp seed- 

 lings, it lasts longer. If the traction is increased after the growth rate begins to 

 surpass that of the control, a second period of retardation ensues, as is seen in the 

 case of the Dahlia shoots,where the weight was increased at the beginning of the 

 third day, from 50 g. to 100 g. 



Traction is effective to modify the anatomical structure of plants as well 

 as to produce alterations in the rate of enlargement. 



Our knowledge of the effect of pressure upon plant growth has been much 

 advanced by the work of Pfeffer, 1 who embedded growing plant parts in plaster 

 of Paris or gelatine, and studied the pressures developed by growth, and their 

 effects upon the tissues. According to the problem in hand, either the entire 

 plant or just the growing region was embedded. Plaster of Paris proved very 

 satisfactory in these experiments, since it furnishes a rigid material when it 

 hardens and at the same time allows free access of both air and water to the 



! Pfeffer, W., Druck- und Arbeitsleistung durch wachsende Pflanzen. Leipzig, 1893. 



