80 MORPHOLOGY OF SEXES. [BOOK i. 



and rendering more equable the pressure on the interior 

 circles, so as to allow of the growth of parts which are usually 

 abortive. (Annals of Natural History, vol. iv.) 



E. Change of Sex under the influence of external causes. 

 Mr. Knight long ago showed that a high temperature favoured 

 the development of male flowers, and a low one that of 

 females. 



In a forcing-house, a fire of sufficient power only to pre- 

 serve in the house a temperature of about 70, during summer, 

 was employed, but no air was ever given, nor its escape facili- 

 tated, till the thermometer, perfectly shaded, indicated a tem- 

 perature of 95 ; and then only two of the upper lights, one 

 at each end, were let down about four inches. The heat of 

 the house was consequently sometimes raised to 110, during 

 the middle of warm and bright days, and it generally varied, 

 in such days, from 90 to 105, declining during the evening 

 to about 80, and to 70 in the night. Late in the evening of 

 every bright and hot day, the plants were copiously sprinkled 

 with water, nearly of the temperature of the external air ; and 

 the following were the effects produced upon the different 

 species : 



A plant of the Water Melon grew with health and luxu- 

 riance, and afforded a most abundant blossom ; but all its 

 flowers were male. On the other hand, Mr. Knight had 

 many years previously succeeded, by long-continued very 

 low temperature, in making Cucumber plants produce female 

 flowers only ; and he entertained little doubt that the same 

 fruit-stalks might be made to support either male or female 

 flowers, in obedience to external causes. In like manner, 

 when Strawberry plants are subjected to a high temperature 

 in forcing-houses, they produce male flowers; and females 

 only in a comparatively low one. 



It would seem, however, that other external causes, beyond 

 mere heat, influence the production of males or females. In 

 the Ray Reports it is mentioned that M. Hampe observed, 

 in a bush of Salix repens, that twigs above the water blossomed 

 as females, whilst those twigs which had been in the water, 

 and subsequently blossomed, when the water was dried up 



