DIFFUSION OF FLUIDS. 571 



The relation of the composition of the fluids of the plant to the amount 

 of transpiration has lately been studied by M. Burgenstein, by growing 

 plants in saline solutions, taking care to prevent all evaporation except from 

 the leaves of the plant, and then weighing the apparatus and plants daily 

 so as to estimate the loss. Acids were found to increase the evaporation, 

 alkalies to diminish it, other things being equal. Saline solutions act vari- 

 ously, according to the nature of the salt and the concentration of the solu- 

 tion,transpiration attaining a maximum at a certain degree of concentration 

 and diminishing after that. In a mixed saline solution, or complete nutri- 

 tive fluid, however diluted, the quantity of water transpired is invariably 

 less than when the plants are grown in distilled water. 



Tissues through which the Sap flows. As regards the special 

 tissues through which the sap flows, the experiments of Hoffmann 

 and others indicate that a very uniform diffusion of fluids takes 

 place in the Cellular plants and in the Mosses. But the last- 

 named physiologist found that in the plants possessed of fibro- 

 vascular bundles, the fluids passed up in the first instance from 

 the roots chiefly in the prosenchymatous cellular constituents or 

 soft bast-cells of the bundles. These experiments were made by 

 causing the plants to absorb potassic ferr.ocyanide ; and then, by 

 treating sections of them with a per-salt of iron, the course of the 

 sap was shown by the local appearance of Prussian blue. 



Unger's experiments, in which he caused plants to absord the red juice 

 of the berries of Phytolacca, gave the same results. As a rule, it was found 

 by both observers that the fluids did not pass by the spiral vessels them- 

 selves, unless the continuity of the absorbing surface of the roots had been 

 destroyed. Herbert Spencer's experiments, however, show that the passage 

 through the vessels is much more rapid than through the cellular tissue. 

 Where cut branches are caused to absorb, the fluids rise in the open vessels 

 and ducts by simple capillarity. In McNab's experiments the ascending 

 current was found to pass only through the woody portion of the nbro-vas- 

 cular bundles and not through the liber. 



The spiral and other vessels do not always participate in the diffusion of 

 the juices ; but in the commencement of the growing-season (with us, in 

 spring), the whole tissue becoming gorged with fluid, the vessels are com- 

 monly found full of sap. In the regular steady course of vegetation the 

 spiral vessels are usually found filled with air. 



The intercellular passages are also filled with air, except under peculiar 

 circumstances, and therefore take no part in the distribution of the sap. 



The experiments which have been made to ascertain the course of the 

 fluids absorbed by the roots, tend to show that the sap passes upward in 

 the elongated cells associated with vessels in the fibro-vascular bundles, 

 towards and into the leaves and other organs. The distribution of the 

 fluids must therefore be very different in stems differently organized as 

 regards the arrangement of these bundles. In Monocotyledons we find a 

 series of isolated streams ; in Dicotyledons the fluids ascend in a much 

 freer and wider course, in the more abundant wood of the regularly ar- 

 ranged circle of bundles. A further diversity arises from the changes which 

 take place in stems with age : in Dicotyledons the inner layers of wood 



