180 PHYSIOLOGICAL BOTANY. PART II. 



placing a branch in coloured fluids, such as a decoction 

 of Brazil-wood or cochineal, they are absorbed and the 

 course of the sap through its whole passage into the 

 leaf may be readily traced ; but on examining micro- 

 scopically the stains which have been left, it is scarcely 

 possible to feel satisfied whether they are on the outer 

 or inner surface of the vessels and cells which they have 

 discoloured. The mutilated state of the stem, when 

 subjected to experiments of this description, lias also 

 introduced errors into the results, and the coloured 

 liquids have been observed to rise up certain vessels 

 which under ordinary circumstances appear destined to 

 convey air. Since there are many plants which possess 

 no vascular structure, in them at least we must allow the 

 cellular tissue to be the true channel through which the 

 sap is conveyed. But whatever may be the manner in 

 which the effect is produced in the more succulent parts 

 of plants, it seems to be unquestionable that a more di- 

 rect mode of progression than that of a gradual trans- 

 mission from cell to cell, must exist in the older parts 

 of woody stems. If for instance we take a long branch 

 of the vine and bend it in the middle, the sap imme- 

 diately exudes at the extremities, but chiefly on those 

 sides which are towards-the concave surface produced by 

 the flexure; which not only indicates a continuity, but 

 also a rectilinear course in the channels through which 

 the sap is conveyed. It is further evident that a general 

 intercommunication must subsist between these several 

 channels ; for the stem may be notched to the very 

 centre, at different altitudes and on different sides, so as 

 completely to intercept every r.ectilinear communication 

 between the lower and upper parts, and the sap will 

 still find its way into the leaves. The probability 

 therefore seems to be, that the crude sap really rises, 

 at least in woody stems, through the intercellular pas- 

 sages, where it bathes the surface of the cells and ves- 

 sels, all of which are so many distinct organs destined 

 to act upon it and more especially when it has after- 

 wards become intermixed with the proper juices of the 



