342 Plant Life and Evolution 



of true graft hybrids, which were almost exactly 

 intermediate in all respects between the nightshade 

 and tomato. 



EXPERIMENTAL MORPHOLOGY 



Much attention has been given of late to the ex- 

 perimental study of the formative effects of environ- 

 ment upon the developing organism. Both zool- 

 ogists and botanists have become much interested 

 in this question, and many important works have 

 appeared during the past few years. While caution 

 is necessary in deducing from the results of these 

 artificial experiments, the laws governing the devel- 

 opment of organisms under normal conditions, nev- 

 ertheless much light has been thrown, by these 

 experiments, upon some of the fundamental prob- 

 lems of evolution. 



One fact stands out especially prominent, namely 

 the remarkable plasticity of the plant-organism, 

 which responds readily to a very great variety of 

 stimuli and shows an extraordinary range of varia- 

 tion within the species. These experimental studies 

 also demonstrate very forcibly the generalized char- 

 acter of the plant tissues, and the readiness with 

 which one organ may take over the functions of 

 another when it is necessary. Space forbids more 

 than a very brief reference to a few typical in- 

 stances. 



The leaves of many plants, e.g., roses, peas, are 



