344 Plant Life and Evolution 



Klebs' Experiments on Formative Effects of 

 Stimuli. The various formative factors, light, heat, 

 food, etc., have already been sufficiently discussed 

 and will not be dwelt upon here at length, but it 

 may be worth while to refer briefly to a few cases 

 where the formative effects of some of these stimuli 

 have been critically studied. Klebs for several years 

 has been investigating the direct causes affecting 

 the character of plant structures, and the results of 

 his studies are extremely interesting and valuable. 

 Especially instructive are some of his studies upon 

 the lower organisms, whose simplicity makes it 

 easier to judge the direct effects of the stimuli em- 

 ployed in the experiment. Klebs showed that it is 

 possible for the experimenter to control almost ab- 

 solutely the character of the development of the 

 plant. By the employment of certain stimuli, e.g., 

 light of varying intensity or color, nutritive media 

 of different kinds, etc., the plant can be forced to 

 develop in almost any way the experimenter may 

 select. 



Reproduction of any type can be induced, or the 

 reproductive activity may be entirely suppressed. 

 Klebs has later extended his studies to the higher 

 plants, where, owing to the greater complexity 

 of the organism, the formative factors are not 

 nearly so clearly evident. Nevertheless, in these 

 higher plants, also, he has shown that it is possible 

 to control to an extraordinary degree the develop- 

 ment. By varying the character of the light, tern- 



