LIGHT AND PLANT GBOWTH — BRACKETT AND JOHNSTON 257 



the tissues between these two regions must be capable of conducting 

 the excitation from the former place to the latter. It would appear 

 on first sight that a plant is very much like an animal which, for 

 example, sees food with its eyes. The excitation travels over the 

 nerves to the legs which react in such a manner as to carry it toward 

 the food. In a growing shoot, the tip perceives light on one side. 

 The sensation is transmitted to the part lower down where differ- 

 ential growth goes on in such a manner as to point the stem or shoot 

 toward the light. 



A good deal of work on phototropism has been done with the 

 coleoptiles of plants belonging to the grass family. The coleoptile 

 is a leaf sheath surrounding the plumule or bud of the ascending shoot 

 or foliage leaves. The oat is a favorite plant for such experiments. 

 One ingenious experimenter as recently as 1910 hit upon tlie unicjue 

 idea of cutting off the top of a coleoptile and sticking it back on the 

 stub with melted gelatin. AVhen the tip was illuminated on one 

 side the shoot still showed marked phototropic bending at the base. 

 Did the stimulus received by the tip travel to the base after passing 

 through a layer of gelatin? This experiment stimulated much 

 interest and many curious methods were devised for decapitating 

 the coleoptiles and replacing their " heads " in various positions. 

 Further interesting experiments are reported in which the " heads " 

 of coleoptiles illuminated from one side were cut off and stuck on the 

 stumps of decapitated coleoptiles grown in the dark. These 

 " doctored up " coleoptiles when allowed to continue their growth in 

 the dark showed positively phototropic curvatures in the proper 

 direction. 



Professor Priestley, of England, has recently been studying these 

 very interesting plant traits and has done much toward giving this 

 peculiar phenomenon a rational explanation. He shows that photo- 

 tropic curvature in coleoptiles is consistent with the " light-growth " 

 hypothesis in spite of many seemingly discrepant experiments. The 

 amount of light required to induce phototropic curvature in normal 

 light-grown shoots is greater and must be continued longer than that 

 required to bring about similar curvatures in etiolated shoots. 

 Etiolated shoots are those grown in the absence of light. Such 

 plants are white or pale in color and usually differ from normal 

 plants in the structure of their tissues. Light affects normal and 

 etiolated shoots quite differently as will be shown. 



It is well to pause at this point to consider a few essential condi- 

 tions for growth. The meristematic tissue consists of a group of 

 actively dividing cells. An available water and food supply is 

 necessary for this kind of growth. Extending up and down plant 

 stems are tiny tubes through which water and solutions of food 

 material pass. In order that these materials, so essential for 



