INVERTEBRA.TA, CRYPTOGAMIA, MICROSCOPY, ETC. 299 



logo Farfara, &c. It has always the form of clearly defined grains, 

 occasionally in the epidermal, much more often in the deeper lying 

 cells, slowly soluble in alcohol and potassa. It is in all probability 

 a modification of chlorophyll. Flowers containing xanthine, expand- 

 ing in the dark, do not generally acquire so bright a colour as those 

 whose colouring matter is soluble, the coloration of the former being 

 more directly dependent on light. 



Absorption of Salts through the Roots. * — M. Vesque thus states 

 the results of a series of experiments carried on with a view of de- 

 termining the influence of saline substances on the absorptive power 

 of roots for water. 



1. Under ordinary conditions, i. e. when the plant is supplied 

 with mineral food-materials, distilled water is absorbed more freely 

 than saline solutions or nutrient fluids. 



2. When the plant has been submitted for a longer or shorter 

 time to treatment with distilled water, it absorbs saline solutions and 

 nutrient fluids more freely than distilled water. 



3. A contact, even of short duration, of the roots with distilled 

 water acts favourably on the absorption of salts ; and conversely a 

 temporary contact of the roots with a saline solution promotes the 

 absorption of distilled water. 



4. The efiects are the stronger, the more concentrated the saline 

 solutions and the nutrient fluids. 



5. There is no qualitative diflference between the absorption of a 

 solution of an isolated salt and that of a nutrient fluid. 



Precisely similar results were obtained respecting the absorptive 

 power of branches detached from their roots. 



Equilibrium of the Pressure of Gases in the Tissues of Plants-t — 

 J. Wiesncr proposes to limit the term diffusion to an intermingling 

 of chemically diflbrcnt gases subject to an equal pressure, whether 

 separated or not by partition walls ; while by effusion he uudcsrstands 

 the intermingling of two gases which are chemically alike or unlike 

 but subject to difl'crent pressures, the consequent currents taking 

 place through fine openings in thin membranes. The following arc 

 the general results of a series of elaborate experiments : — 



1. There are tissues which are impermeable to air even under 

 a great inequality of pressure, as periderm destitute of lenticels. 



2. The ingoing and outgoing currents of air tlirongh the stomata 

 are the result of efi'usion ; the times for the passage of a definite 

 volume of gas in such currents being proportional to the square root 

 of the density of the gas. 



3. In non-vascular wood the equilibrium of pressure is effected 

 througli the cell-walls ; it is ([uickcst in the axial, slowest in the 

 radial direction. The delicate nicuibraneof the bordered pit is eitlier 

 the exclusive cliannel, or at all events permits a much freer exchange 

 than tlie rest of the wall. 



4. In parenchyma containing air, a portion of tlie air passes 



* ' Anu. Sci. Nat.,' ix. (1879) p. 1. 



t ' SB. K. Akatl. WisB. Wieu,' l.vxis. (1871)) [u 3GS. 



