THE BIOLOGY OF PLANT GROWTH 445 



which is measuring diurnal cycles. At 25° C, it this is the optimum 

 temperature for the plant, it is measuring off 24-hour periods. During 

 the diurnal cycle the plant appears to consult its clock to ascertain 

 what portion of the daily ritual comes next. The clock says, "Now 

 photosynthesize," or "Now it is night," or whatever it is that plants have 

 to do on a 24-hour basis. At low temperatures the clock by which the 

 plant programs its operations apparently runs a little too slowly. If the 

 clock is not in synchrony with the outside real-world events, our plant 

 suffers metabolic disturbances. In order to cure the plant of these dis- 

 orders, what we have to do is to give him his light and dark on a 

 diurnal cycle of a period in synchrony with that of his clock. The clock 

 runs slowly at low temperatures, and we must therefore give him daily 

 cycles of light and dark which are longer than 24 hours in order to 

 make the plant happy at low temperatures. The clock runs too rapidly 

 at high temperatures, and we must therefore give the plant a diurnal 

 cycle shorter than 24 hours at high temperature. 



This behavior applies to all species we have so far investigated— 

 which, it is true, is a small number, namely three. The species thus far 

 investigated care not only about the absolute temperature but care in 

 addition about the temperature in relation to the diurnal-cycle length. 

 They all behave as though they have clocks which measure time and 

 which must be in phase with the daily external time signals. 



This is a facet of plant behavior about which nothing is known. It 

 is a facet of plant beha\ ior which is basic to our understanding of the 

 relation of plants to temperature. I feel sure that when we find out why 

 the plant has a clock and how this clock works, we will then be able to 

 find materials which we can gi\e to a plant and will say to its clock, 

 "Please run faster, because you are running too slowly," or "Please run 

 slower, because you are running too fast," and that we will in this 

 way be able to do a great deal to foster the mating of plants and 

 temperature. 



The act of photosynthesis produces plant material. The partition 

 of this raw material between the varied organs and structures of the 

 plant is controlled in large measure by the plant hormones, which 

 carr)' out their work as messengers acting under the authority of and 

 in accordance with the instructions contained in the plant's genetic 

 material, and thus, in a third sense, the manifold hormonal systems of 

 plants determine growth and form, vegetation and reproduction in the 

 plant world. We know a great deal about plant hormones. We know, 

 for example, that there are root growth hormones— sul)stances pro- 

 duced in the leaf and transported to the roots, which, though they can- 

 not synthesize these materials, require them in their growth. These 

 root-growth-controlling hormones of the plant are the chemical sub- 

 stances thiamine, niacine, and pyridoxine. We know, similarly, that 



