INTRODUCTION 7 



experiment of Drew ( 1923) , who found that whereas 

 in tissue cultures embryonic kidney cells grow as an 

 undifferentiated mass, the insertion of a bit of con- 

 nective tissue stimulates the kidney cells to differen- 

 tiate. 



Similar effects are to be observed in multicellular 

 organisms; indeed, the humoral transmission of stim- 

 uli already referred to may be regarded as an example 

 of the influence of a chemical substance produced by 

 one group of cells upon another tissue. Such effects 

 have also been observed in plants, particularly in their 

 heliotropic responses. As is well known, green plants 

 bend towards the light. Many years ago, Darwin 

 (1880), experimenting with the seedlings of Phalaris, 

 reached the conclusion that the effect of the light is 

 not upon that part of the plant which actually bends, 

 but is due to a modification of growth which is brought 

 about by a stimulus transmitted from the tip of the 

 plant. This he proved by preventing the access of 

 light to the tip of the seedling, whereupon, although 

 the stem was illuminated, no heliotropic bending took 

 place. 



It is a reasonable assumption that the stimulus is 

 transmitted by means of some substance resulting 

 from a photochemical change induced by the light 

 in the cells at the surface of the tip. In agreement with 

 this concept, Blaauw (1908) found that the helio- 

 tropic response of etiolated oat seedlings conformed 

 to the Bunsen and Roscoe law — namely, that the 

 chemical effect produced by light is proportional to 

 the product of the intensity and the duration of the 

 illumination. 



Further proof of the chemical nature of the helio- 



