Growth and Movement. 113 



cient quantity to render the cells turgid ; food materials 

 must be available ; there must be a certain amount of exter- 

 nal energy in the form of heat ; oxygen must be present 

 for the process of respiration, resulting in the setting free 

 of internal energy, without which the life of the plant 

 would become extinct; finally, there must be an inclination 

 to cell division. This last condition is itself dependent, to 



FIG. 48. 



A, a shoot of Virginia creeper growing in the light. B, a shoot from the same plant 

 growing in partial darkness. 



a certain degree, on the others ; but, even when these are 

 favorable, the inclination to cell division does not always 

 result, as we see in the cessation of growth of hickory 

 shoots in the height of the growing season, and in the 

 limited growth of embryos in forming seeds. The causes 

 for this behavior are inherent in the nature of the cell, and 

 are faithfully transmitted by cell division from generation 

 to generation. 



