Chap, x The Problems of Growth 451 



cerned, with the result that the plants showed all the 

 phenomena of etiolation. Sachs' results were confirmed by 

 Kraus, Vines, Wiesner, and others; many researches on the 

 subject being published in the years 1878-93. 



While it is clear that a certain intensity of illumination 

 is necessary for the proper regulation of growth, many 

 researches showed that the results of excessive insolation 

 are as unfavourable as those of darkness. Wiesner showed 

 that exposure to very intense light arrests growth altogether. 

 Pringsheim observed in 1879 that concentrated light kills 

 ordinary green plants if they are exposed to it for a con- 

 siderable period. Pfeffer, in 1875, and Darwin, in 1880, 

 also observed that exposure to very brilliant illumination 

 is attended by fatal results in the case of many leaves, 

 which however protect themselves from excessive insolation 

 by turning their edges instead of their surfaces to the 

 incident rays. Darwin gave the name paraheliotropism to 

 this phenomenon in 1880. It was explained by Pfeffer as 

 due to the unilateral illumination producing a greater fall 

 of turgor in the more strongly illuminated half of the 

 pulvinus than in the less strongly illuminated half. 



The effects of light upon plants are manifested very 

 widely ; those which are observable upon growth and 

 growing organs are only part of very far-reaching pheno- 

 mena, which were the subjects of much study and 

 research. Those effects which have so far been noticed 

 may be taken to be but instances of what are now known 

 as the tonic effects of light. 



Certain very curious protective mechanisms were dis- 

 covered, having for their object the avoidance of injury by 

 the chloroplastids of the leaves, but their discovery dates 

 back to a period just earlier than that under review ; 

 Boehm noticed in 1856 that the plastids of the leaves »t 

 certain of the Crassulaceae change their position when the 

 light becomes intense. Famintzin and Borodin studied the 



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