OF SAP IN PLANTS. 51 



ting paper, the latter absorbed a great deal of the solution (in the 

 horizontal direction from the wood) even when the bark much 

 lower down was unaffected. Or if only a ring of bark was cut 

 out on the upper part of the shoot, this interruption was no 

 hindrance to the advance of the solution ; it was found at the 

 end, both in the wood and in the bark. The epidermis did not 

 give a blue reaction even after remaining one hour in contact 

 with the salt of iron. 



Salix acuminata, Sm. — The question investigated in this case 

 was, how far a horizontal conduction of the fluids can take place 

 under favourable circumstances in the wood itself, through layers 

 of different ages. For this purpose the point of a small twig 

 was cut off, at the end of June, and the open end immersed in 

 the solution. The main shoot (6 lines thick) which bore the 

 foregoing was so notched circularly in four different places, that 

 there was no immediate communication with the vessels of the 

 stem in any place : the wounds were enveloped in fresh leaves. 

 After eight days it was found that the solution had advanced 

 exclusively in that side of the main shoot which corresponded to 

 the absorbing twig, and indeed only as far as the notch which 

 interrupted the vascular communication 4 inches further up. 

 Here also the outermost layer of wood and the liber had conducted 

 principally, and not the cellular intermediate layer of the bark. 

 From this we see what difficulties are opposed to the assumption 

 of a horizontal movement of the sap through the medullary rays; 

 although, at the same time, a horizontal movementof the sapsin 

 the young wood, generally, under very favourable circumstances, 

 as in the experiments of Hales (/. c), cannot be disputed. 



Postscript. — With regard to the behaviour of the milk-sap, 

 which I had an opportunity of observing in several EuphorhvB, 

 in Sonchus oleraceus, &c., in reference to the conduction of sap, 

 I am led to assume, from all that I could notice, that it takes no 

 part whatever in this, whether the fluid penetrate into the plant 

 through the leaves or through the roots, setting aside all the 

 anatomical reasons against a circulation of the milk-sap, the most 

 decisive of which is, that in the majority of plants the milk-sap 

 passages have no continuity or general distribution. [A.H.] 



4* 



