300 PERMEABILITY IN PLANTS 



In Ulva the structure of the frond is much simpler than in the 

 other plants studied. It consists of two layers of similar cells, form- 

 ing a membrane about 0.078 mm. in thickness. The cell walls are thin 

 and consist of cellulose.^ As it is not sufficiently stiff to be treated 

 like Lammaria, pieces were cut out and supported between disks of 

 hard rubber and celluloid as described in a previous paper. ^ 



The leaves of Zoster a have a very simple structure. The fibro- 

 vascular bundles are poorly developed. There are intercellular spaces, 

 but it is possible to find leaves in whix:h there is very little gas in 

 these spaces. In such material the amount and position of the gas 

 seems to remain unchanged during the experiment. The cell walls 

 are of cellulose. 



As the leaves are not wide enough to furnish disks such as were 

 used in other experiments, a different procedure was adopted. A 

 number of pieces were held in a horizontal position (parallel and a 

 little separated) by means of two parallel celluloid combs placed at 

 right angles to the length of the leaves. The electrode carriers moved 

 between the combs, forcing the leaves together when measurements 

 were made. The electrode carriers were held in frames as in the case 

 of Laminaria, but the two upper glass rods were removed and a 

 special form of hard rubber plate was placed at each end. 



In some of the experiments pieces of the leaf were placed between 

 disks of hard rubber and celluloid, just as in the case of Ulva. 



In all cases the resistance was first determined in sea water, which 

 was then replaced by another solution of the same conductivity. All 

 readings were made at the same temperature or, if necessary, cor- 

 rected to the proper temperature. 



Experiments on Laminaria have shown that it is possible to follow 

 the progress of death in the same manner as the progress of chemical 

 reactions in vitro. For the most part the process follows more or less 

 closely a monomolecular course. For Laminaria placed in NaCl 

 0.52 M, the velocity constant of the reaction at 15°C. was found (as 

 the average of eight experiments) to be 0.00723. For Rhodymenia 

 the same number of experiments (at the same temperature) gave 

 K = 0.0009. Hence the rate appears to be about eight times as great 



^ The wall contains a methyl pentosan yielding rhamnose. Cf. Czapek,^ 641. 

 * Osterhout, /. Biol. CJiem., 1918, xxxvi, 563, fig. 3. 



