188 



PLANT PHYSIOLOGY 



suspended from threads passing over pulleys and fastened to 

 growing parts. The following figures will illustrate the 

 results obtained 



Subjecting otherwise weak stems to pull induces them to 

 form strengthening tissues which would not ordinarily de- 

 velop at all. Stems thus acted upon decrease their growth 

 in length in proportion as they are stimulated to grow in 

 thickness. Even a strain too slight to produce any stretch- 

 ing will have this effect. 



The formation of strengthening tissues is proportional to 

 the need. This is shown each year by fruiting plants. The 

 fertilization of the egg-cells in the ovules of flowering plants 

 leads to the production of seeds, the growth of fruit, and 

 the considerable increase in weight of these and of the adja- 

 cent parts. The development of the fruit and its contents 

 demands a corresponding growth both of conducting and 

 also of mechanically strengthening tissues extending into 

 regions quite distant from the fruit as well as in those near 

 it.* A similar response to mechanical strain is shown by 

 many mosses and liverworts. The stalks bearing the fruits 

 grow greatly in length for a time. Later, when the fruits 

 increase in weight, the stalks cease to elongate and become 

 much thicker and stronger. 



Plants are everywhere in nature exposed to mechanical 

 strains of more or less force and constancy. The winds, 

 flowing water, tides, waves, and the movements of animals 



*Pieters. A. J. Influence of fruit-bearing on the development of me- 

 chanical tissue in some fruit trees. Annals of Botany. X. 1896. 



