2 HOFFMANN ON THE CIRCULATION 



regarded as a secretion, or as a circulating nutrient fluid analo- 

 gous to blood. Even the function of the air-vessels is not 

 clearly made out, or, otherwise, the recent statement, that the 

 " so-called air-vessels " of the Ferns do not convey air, could not 

 have been made. Under such circumstances, it can only be 

 expected that the imperfect knowledge of the actual facts must 

 leave the cause of the movement of the sap in utter uncertainty; 

 and, indeed, on this very point the most wonderful notions are 

 current, capillarity, contractility, and endosmose have to do 

 duty in turn, as far as they will go. 



Where is the crude nutrient sap elaborated ? How does it 

 arrive there ? By what path is it carried back to the other parts 

 of the plant to furnish the material for development ? In what 

 anatomical and physiological relation do the air-vessels stand to 

 the system in which the saps circulate ? 



Injections cannot be applied in the endeavour to trace the 

 paths by which fluids penetrate the vegetable tissues, on account 

 of the minute size of the vessels, nor, indeed, without destruc- 

 tion of the substance. But the spontaneous absorption of easily 

 detected fluids accomplishes in plants what is done by injection 

 in human anatomy. Coloured fluids, however, are rarely taken 

 up by uninjured roots ; I have therefore made use of a very 

 dilute solution of ferrocyanide of potassium, which may be readily 

 detected in any spot to which it has penetrated, by the blue 

 colour it assumes when chloride of iron is applied. And as the 

 Prussian blue thus formed is insoluble in aqueous fluids, a little 

 care in slicing and preparing the objects enables us to avoid the 

 spreading of the colour to unaffected parts, which would deceive 

 the observer as to the boundaries within which the natural mo- 

 tion of the sap takes place. 



In the first place, it is found that this fluid penetrates by a 

 different path in uninjured roots, from that which it takes when 

 the solution is caused to be absorbed directly by the cut surface 

 of a cut plant ; in the next place, that the course which this 

 fluid pursues is constantly the same, and peculiar to each plant; 

 and, moreover, that by no means all cells and vessels take equal 

 share therein, but that where, in general, vessels are met Avith, 

 the sap enters first into these,, and then, as in animals, passes far 

 more slowly from these into all the remaining tissues of the plant. 



