378 LECTURE XVI. 



time is smaller when the temperature is made to vary fre- 

 quently between two points, than when it is constant at the 

 mean between these two points. But Pedersen has found 

 that this is not the case, but that when the higher tem- 

 perature was not allowed to exceed the optimum by many 

 degrees, variations of temperature produced no perceptible 

 effect upon the growth of the roots with which he experi- 

 mented. Pfeffer has investigated the matter with regard to 

 leaves, and has found that these, too, are unaffected in their 

 growth by variations of temperature. But in his researches 

 on the opening and closing of flowers, Pfeffer found that in 

 some cases the perianth-leaves were very highly s'entitive 

 to a variation of temperature. The flowers of Crocus vernus 

 and of Tulipa Gesneriana opened under the influence of a rise 

 of temperature and closed under the influence of a fall. This 

 only took place, however, within certain limits of tempera- 

 ture. In Crocus vernus, for example, a rise of temperature 

 produced no opening-movement until a certain minimum 

 temperature, about 9 C., had been reached ; at a relatively 

 high temperature, about 27 C., the opening ceased, and on a 

 further rise, at 28*6 C., closing began but was usually incom- 

 plete : at 367 C. all movement ceased : a fall of temperature 

 produced in all cases a closing-movement. 



Pfeffer clearly established, in the first instance, that the 

 opening and closing of flowers is a phenomenon of growth, 

 and moreover of heterauxesis, the movement being due in 

 either case to the unequal growth of the two surfaces of the 

 dorsiventral perianth-leaf. This heterauxesis may be induced 

 entirely by the variations of temperature, for when a flower 

 is kept in darkness and at a constant temperature, it does 

 not open or close. The relation between the two surfaces, 

 during opening or closing, is this, that only the one side 

 grows perceptibly; thus, in opening, the upper or inner 

 surface grows, whereas the lower does not grow at all or only 

 very little, and conversely, in closing, the lower or outer 

 surface grows and the upper scarcely grows at all. The 

 dependence of these movements upon the stimulating action 

 of variations of temperature has been clearly brought out by 



