6 HOPFMANX ON THE CIRCULATION 



imperfect conduction of the sap in these places. The fruit-stalk 

 absorbed only a small portion of the fluid extremely slowly ; 

 when cut across, and the cut surface dipped in the solution of 

 ferrocyanide of potassium, it soon exhibited a very distinct re- 

 action ; strongest below, where rind and pith were coloured ; 

 weaker above, where only the pith vvas acted on (and this 

 slightly). 



From this it appears that the passage of the sap goes on 

 chiefly and most energetically in a peculiar layer of cells along 

 the margins of the leaves, in these mosses, while the mid-nerve 

 fulfils another function ; I could not detect air in the latter. The 

 stem and the fruit- stalk conveyed fluid onward through all their 

 tissues, but very slowly so long as they remained uninjured. 

 When chloride of iron was applied to the uninjured rind, it pe- 

 netrated extremely slowly, which is sufficiently explained by the 

 solidity of the structure. 



Hypnum cupressiforme, L. — In this there are no peculiarly- 

 formed marginal cells and no mid-nerve ; and no definite direction 

 of the course of the sap could be detected here. This plant 

 became gradually saturated with the fluid, so that even the 

 peristome was coloured blue. 



4. Ferns. 



Pteris serrulata, — The test fluid w^as dropped upon the mould 

 in which the plant was rooted, without wetting or injuring the 

 latter. In twenty-four hours it had already penetrated far up 

 the petiole ; the reagent did not colour the brown internal layer 

 or the vascular bundle in the central point ; the latter was white, 

 surrounded by brown soft cells. The parenchyma was blue and 

 rich in granules (chlorophyll and starch?) throughout. The 

 scalariform vessels in the interior distinctly contained air, and 

 when uninjured took up no test fluid, although the plant was in 

 a most vigorous condition of growth. 



Poly podium crassifolium, — Treated like the preceding. In 

 twenty-four hours the petiole was permeated by the fluid, and both 

 the rather delicate cortical layer (of a green colour), containing 

 no chlorophyll, and the entire parenchyma, which contained 

 very numerous granules, were coloured blue by the reagent. 

 The latter is traversed in the inner parts by isolated vascular 



