180 BOARD OF AGRICULTURE. 



cells are almost exclusivel}^ colloids, aud the proof of their 

 easy and rapid transmission through imperforate memhranes 

 is yet to be discovered ; neither is there sufficient evidence of 

 any such exudation of organic matter from the rootlets, where 

 osmose is imagined to occur, as is required by all that is 

 known of this principle in its operation upon lifeless matter. 



Ordinary absorption and capillary attraction have been 

 thought to assist in producing the phenomena of the motions 

 of sap, though no one regards them as sufficient of them- 

 selves, since they not only lack the requisite power, but also 

 that peculiar ability, manifested by the living plant, to select 

 from the soluble materials of the soil just those substances 

 which every specie^ needs for its peculiar constitution. 



Herbert Spencer has attempted to demonstrate that the 

 compression and dilatation of cells and ducts, caused by the 

 swaying of stems and branches in the wind, is an important 

 aid in promoting the flow of sap. When we consider that 

 many trees grow where the wind scarcely affects them, and 

 that plants flourish in glass-houses, where they are never dis- 

 turbed in this way, we shall see that this hypothesis is of • 

 small account. There is also here, as in the hjqDothesis of 

 Malpighi, a need of valves to prevent regurgitation, and we 

 have, during the present season, demonstrated that detached 

 living roots, entirely underground, exert an enormous force 

 merely by their power of absorption. 



After this general discussion concerning the circulation of 

 sap in plants, we are prepared to consider in -a very brief 

 manner the results of a few experiments instituted for the 

 purpose of asking the trees a few questions which the books 

 did not satisfactorily answer. 



The earliest investigations in this direction of which we 

 have a record were begun about the year 1720, by Rev. Ste- 

 phen Hales, an English clergyman, and published in a vol- 

 ume entitled, "Statical Essays, containing Vegetable Stat- 

 icks ; or. An Account of some Statical Experiments on the 

 Sap of Vegetables ; being an Essay towards the Natural His- 

 tory of Vegetation. Of use to those who are curious in the 

 Culture and Improvement of Gardening, &c., &c." 



For the first experiment described, he took a flower-pot in 

 which was growing a sun-flower, three feet and a half in 



