THE AMERICAN BOTANIST 69 



completely perfect flowers to flowers that are either staminate 

 or carpellate. Ten different forms have been found in the 

 ash. According to a writer in the American Journal of Bot- 

 any, more than ninety families of plants have species that 

 show differences of the kind mentioned. 



Heat and Fruiting. — It is a matter of common knowl- 

 edge that plants must have light to make plant food, but ac- 

 cording to two scientists of the Bureau of Plant Industry, 

 U. S. Department of Agriculture, the fruiting of many, 

 perhaps all, plants may be controlled by regulating the period 

 of light to which they are exposed. It is reported that either 

 too long or too short a period of daylight in comparison with 

 the darkness may prevent plants from flowering or fruiting. 

 In case the period of light is unfavorable, the plants may make 

 a luxuriant growth but fail to fruit. A length of day that is 

 favorable to both vegetation and fruiting is assumed to pro- 

 duce the "everbearing" varieties. It has been known for a 

 long time that plants have three cardinal temperature points 

 or zeros — an upper and lower zero beyond which growth 

 ceases and a middle or optimum zero at which they thrive 

 best. Frequently these zeros are different for the growing 

 and flowering processes. Undoubtedly the upper and lower 

 zeros are associated with temperature, but it may well be that 

 the optimum point is determined more by light. In view of the 

 experiments, one understands how apple trees may fail to 

 fruit when taken to a warmer region, not because of the heat, 

 but because the period of daylight is not favorable. A curious 

 result of this rule is found in the case of the ragweed which 

 is reported to require for flowering a stimulus that is afforded 

 by the shortening of the days and lengthening of the nights. 

 It does not come into flower until the period of daylight falls 

 below 15 hours. In the latitude of Washington, that comes 

 about July 1. But if ragweed seed should be taken to north- 



