18 IF. J. Robbins 



to as the more mature years is the privilege it gives of looking back 

 and seeing the changes and advances which have occurred in a life- 

 time. Plant physiology in my graduate student years, more than 40 

 years ago, consisted mainly of a consideration of mineral nutrition, 

 transpiration, water requirements, osmosis and osmotic pressure, 

 photosynthesis, nitrogen relations, especially nitrogen fixation, respir- 

 ation, toxicity, antagonism, and balanced solutions, conduction of 

 water and translocation of organic food, and similar fundamental pro- 

 cesses. Any phenomenon not readily explainable on some other basis 

 was assigned to that universal solvent of all problems, changes in 

 permeability. That naturally occurring specific organic substances 

 in minute amounts might materially influence the growth and de- 

 velopment of a plant had little evidence to support it and received 

 scant attention from teachers, students, or investigators. 



It is true that in 1858-1860 Pasteur had observed that the develop- 

 ment of yeast and of lactic acid bacteria in a synthetic medium was 

 favored by the addition of small amounts of natural products. Sachs 

 (17) had proposed the concept but not the term hormone. Wildiers 

 (26) reported that minute amounts of Bios, a concentrate of unknown 

 composition, was of great importance for the growth of some races of 

 yeast. Ludwig Jost, in his Plant Physiology (7) was quite prepared to 

 account for the formation of insect galls by the action of some definite 

 substance which diffuses out from the larva and stimulates the cells 

 to hypertrophy. Loeb (8) had suggested that hormones produced by 

 the leaf of Bryopliyllum played a role in the formation of new plants. 



But these reports and others like them could be and were ex- 

 plained on some other basis than the action of specific compoiuids. 

 It required something as decisive and dramatic as the auxins and their 

 effects to change the viewpoint of plant physiologists interested in the 

 problems of growth and development. 



For some years the auxins were the only growth regidators to which 

 botanists devoted much serious attention. But new knowledge of the 

 existence of vitamins, hormones, and similar substances, their chem- 

 istry and their action in animal physiology, together with the discov- 

 ery that there were some problems in the develo{)ment of plants not 

 solved by auxins alone, led to a substantial expansion of our concepts 

 of plant growth regulators. 



Schopfer's discovery (18) that Pliycomyccs blakesleeanus required 

 for growth an external supply of small amounts of thiamin and in- 

 vestigations on the growth retjuirements of yeast led to the inclusion 

 of the vitamins, the purine and pyrimidine bases, and specific amino 

 acids among the growth regulators. Kinetin (9) and the gibbercUins 

 have been added more recently. 



