EFFECTS OF LIGHT INTENSITY 739 



growth form and that this substance will react on etiolated plants to make 

 them resemble those grown in the light. 



Trumpf (114) repeated this experiment and pointed out that the 

 osmotic concentration of the expressed sap of normal plants is much 

 higher than that of etiolated plants. When the sap of normal plants 

 was diluted to the same osmotic concentration, it caused even less retarda- 

 tion of growth than that of etiolated plants. The sap of both types of 

 plants inhibited the growth of etiolated seedlings and the extent of this 

 inhibition varied directly with the concentration of the sap in the culture 

 solution. 



Trumpf 's (113) experiments clearly demonstrate that etiolation is 

 not a result of lack of chlorophyll, or lack of photosynthesis, since it was 

 possible to produce plants normal in form with no chlorophyll. Further- 

 more, his experiments (114) demonstrate that the action of light is local 

 and that this effect cannot be transferred from an illuminated organ to 

 one remaining in darkness. 



Priestley and Ewing (78) and Priestley (77) have shown that there 

 is an accumulation of protein and fatty substances in the meristem which 

 renders it impermeable to the nutrient sap, and they have concluded that 

 etiolation is a result of nutritional disturbances in the meristematic tissue. 

 This impermeabihty of the protoplasm (cf. Lepeschkin, 58) of meriste- 

 matic tissue in darkness can explain many of the phenomena connected 

 with etiolation; however, root and stem growth take place in darkness 

 in a manner similar to that in light. 



Finally, there is the question of the usefulness of etiolation to the 

 plant. It is evident that excessive stem elongation, together with 

 positive phototropism or geotropism does serve a very useful purpose in 

 elevating the growing point from darkness to a point where it may enjoy 

 favorable light conditions. This reaction is valuable in enabling the 

 plant to emerge from among obstructions in the soil as well as to attain 

 quickly a position favorable for photosynthesis. The failure of leaves 

 to expand in darkness is an economy in the use of reserve foods, particu- 

 larly carbohydrates. However, in many cases no useful purpose what- 

 soever appears to be served by etiolation. 



THE EFFECT OF LIGHT INTENSITY UPON PLANTS 



The influence of light intensity upon plants cannot be considered 

 independently of the time during which any given intensity acts. For 

 instance, as pointed out in the foregoing section, Trumpf (113) has shown 

 that the stage in morphological development attained by etiolated bean 

 plants exposed to light depended upon the total amount of light received. 

 Chlorophyll formation, on the other hand, was greater when the plant 

 was exposed for long periods at low intensity than when exposed to the 

 same total amount of light at higher intensities. Likewise, tests on the 



