LIGHT AND PLANT GROWTH BRACKETT AND JOHNSTON 259 



apex or tip. In tiiis lejjion the cells are rapidly enhirjrinp by tak- 

 ing in water. The water lills up a space in the cell's interior called 

 the vacuole. Such vacuolating cells grow rapidly by enlarging. In 

 their earlier stage, when forming the meristematic tissue, growth 

 proceeded by rapiil division of the cells. Negative phototropic 

 bending of these roots is attributed to the increased rapidity with 

 which these vacuolating cells enlarge under the inlluence of light. 

 Tiiose on the shaded side of the root enlarge less ra[)idly. 



On the basis of the above theory it may be worth while to examine 

 the results obtained with decapitated coleoptiles. These organs, at 

 the stage they are used in such experiments, grow entirely by cell 

 enlargement and not by cell division. Light increases the rate of cell 

 enlargement but the final size is less. The veins or water pipes 

 running to the tip of the coleoptile, terminate in a pore. When 

 the shoot becomes gorged with water this pore serves as a safety 

 valve and frequently a drop of water is seen on the tip of this shoot. 

 If the water pressure is decreased in the pipe line on one side, growth 

 on that side is retarded. Light, by its photochemical action on the 

 proteins and fats, makes the passage of water through the coleoptile 

 tissue comparativeh' eas}', hence when one side is illuminated the 

 flow of water through the vein in that region is facilitated, thereby 

 reducing the turgor or water pressure. This in turn retards growth 

 on the brighter illuminated side. On the less lighted side growth 

 is faster. This causes the shoot to bend toward the light. By cut- 

 ting off the tip, water is freely lost and growth retarded. Now if 

 half the stub is covered so that the veins in that region are blocked, 

 bending due to increased rate of growth will occur even in darkness 

 in such a manner that the blocked veins are on the convex side of 

 the curved shoot. It seems a likely hypothesis that the mechanics 

 of phototropism is a light-growth reaction based to a large extent 

 on the relation of growth to available water and food supply. 



There is little doubt but that the wave length, or color of light, 

 is an important factor to be considered in a study dealing with funda- 

 mental problems of plant growth. Since it appears that photo- 

 tropism is a specific case of light-growth reactions, it is believed that 

 some of the underlying principles relating to the growth of organ- 

 isms, especially plants, can be worked out from accurately controlled 

 experiments dealing with phototropic responses of etiolated shoots. 

 Such shoots are extremely sensitive to light of relatively low in- 

 tensity and short exposure. Before the wave-length effects of light 

 can be determined accurately, it is, of course, necessary to eliminate 

 all intensity eifects. This phase of light and growth is one of the 

 problems being investigated in the laboratories of the Smithsonian 

 Institution. 



