26 PHENOMENA OF PLANT-LIFE. 



by a slight downward current, as happens in endosmose and 

 exosmose. As, moreover, the spongioles of the roots and 

 the leaves are analogous structures, and certain tubes are united 

 in the roots, the downward current in autumn is accompanied 

 by a slight upward current. This accounts for the fact that 

 at all periods of the year, the upward, downward and trans- 

 verse currents exist ; the upward and downward currents be- 

 ing most vigorous in spring and autumn, and scarcely percep- 

 tible in winter. Furthermore, as some of the vascular expan- 

 sions in the leaves are free to absorb moisture, etc., in the 

 same way that the spongioles are, it follows that the general 

 circulation may receive an impulse from the leaves or from the 

 roots, or both together, the circulation going on in a continu- 

 ous current in certain vessels." 



This original effort of the learned lecturer on physiology, 

 at Surgeons' Hall, in Edinburgh, published in 1874, to explain 

 some of the most difficult problems of vegetable life by a 

 mere hypothesis, which assumes that sap flows in the vessels ; 

 that there are spongioles in the leaves which absorb water ; 

 that the sap descends to the roots and escapes from them in 

 autumn ; and that an imaginary system of syphons does all 

 these wonderful things, which have not been proved to occur 

 at all, and which well-informed physiologists are almost 

 unanimous in denying, reminds us of the adage that "a 

 prophet is not without honor save in his own country." This 

 is not the method of the Baconian philosophy. 



In the observations which follow, we hope to add some new- 

 facts to the knowledge of the world concerning the phenom- 

 ena of plant growth ; but are painfully conscious of the need 

 of much more investigation before a complete and correct 

 theory of the circulation of sap can be stated. Exceptions 

 have been taken to the use of the expression " circulation of 

 sap " ; but since there is an evident distinction between the 

 crude and elaborated saps, both in their composition and 

 their location in the plant, at least in the higher forms of 

 vegetation, and since the circulation of blood is accepted as 

 a proper term even when applied to animals without a heart, 

 we prefer to retain it in our vocabulary. 



In regard to the causes which induce the absorption of 

 water and soluble substances by the roots of living plants, it 



