J3O PROCESS OF NUTRITION. CHAP. III. 



blossoms and mature their fruit, even in the midst 

 of winter, when the temperature is at the lowest. 

 And in the case of submarine plants the temperature 

 can never be very high ; so that although heat does 

 no doubt facilitate the ascent of the sap by its ten- 

 dency to make the vessels expand, yet it cannot be 

 regarded as the efficient cause ; since the sap is 

 proved to be in motion even throughout the whole 

 of the winter. Du Hamel endeavours, however, 

 to strengthen the operation of heat by means of 

 the influence of humidity, as being also powerful 

 in promoting the ascent of the sap, whether as re- 

 lative to the season of the year or time of the day. 

 The influence of the humidity of the atmosphere 

 cannot be conceived to operate as a propelling cause, 

 though it may easily be conceived to operate as 

 affording a facility to the ascent of the sap in one 

 way or other; which under certain 'circumstances 

 is capable of most extraordinary acceleration, but 

 particularly in that state of the atmosphere which 

 forebodes or precedes a storm. In such a state a 

 stalk of Wheat was observed by Du Hamel to grow 

 three inches in three days ; a stalk of Barley six 

 inches ; and a shoot of a Vine almost two feet ; but 

 this is a state that occurs but seldom, and cannot be 

 of much service in the general propulsion of the sap. 

 Linnaeus. On this intricate but important subject Linnaeus 

 appears to have embraced the opinion of Du 

 Hamel, or an opinion very nearly allied to it ; but 

 does not seem to have strengthened it by any new 



