SECTIONAL TRANSACTIONS.— K. 385 



Mr. John Walton. — A Review of the Present Position of Knowledge of 

 Palceozoic Bryophyta, with Descriptions of some New Types. 



Mosses.— There are only two reliable records of fossil-mosses of Palaeozoic age— 

 Miisciles polytrichaceus, Ren. and Zeill.,and Muscites Bertrandi, Lignier. Both records 

 are from the upper carboniferous of France. The former consists of small shoots with 

 leaves in the form of an incrustation on shale. The habit and the size of the leaves 

 are the only evidence that we have of its Bryophytic affinity. The latter consists of 

 a petrifaction of a small axis bearing multicellular uniseriate hairs which have oblique 

 cross walls. The centre of the axis is not preserved, and the hairs are attached to 

 the outermost of two or three layers of thick walled cells. The obliquity of the 

 cross walls is very strong evidence for considering them to be rhizoids and for regarding 

 the fossil as a moss stem. 



Liverworts. — In addition to the liverworts described from the upper and middle 

 coal measures of this country (Walton, Ann. Bot., vol. xxxix, 1925), another type has 

 been discovered which has a well-defined vascular strand differentiated from the 

 rest of the tissue of the thallus, which was flat and ribbon-like and branched 

 dichotomously. Since we have only vegetative structures to deal with, we cannot 

 draw any certain conclusions about the relation of these fossil liverworts to the main 

 divisions of the living ones. It is of interest to note, however, that as regards thallus 

 organisation they are but little behind the living ones. Of leafy forms there is one 

 representative, Hepaticites Kidstoni, Walt., which has almost as sharp a differentiation 

 into axis and leaves as those liverworts which are classed in the Acrogynoz. There 

 are several thalloid species, Hepaticites Langi, H. Willsi, and H. vascularis (MS.). 

 The first two have a simple parenchymatous dichotomously branched thallus, while 

 the last has a vascular strand which forks with the forking of the thallus. Hepaticites 

 lobatus has a lobed thallus with a slightly differentiated axial region. 



There were present, therefore, in Palaeozoic times, as at the present day, liverworts 

 with (a) differentiation into axis and leaves, (6) thalloid form with lobed margin, 

 (c) simple dichotomously branched thallus, (d) thalloid form with highly differentiated 

 vascular strand. 



Dr. G. W. Scarth. — The Regulation of Stomatal Behaviour. 



(i) A brief historical statement. 



(ii) A summary of the principal findings already published by the writer regarding 



the relation of stomatal behaviour to the H-ion concentration of the guard cells 



studied chiefly by mounting leaf sections in penetrating acid or alkali. 



It was found that stomata open both in acid and alkali, and that in the latter 

 there are various intercellular changes which duplicate those associated with normal 

 opening in light. Evidence was also discovered that the pK in the guard cells influences 

 their turgor partly through its effect on the ' swelling ' capacity of a colloid in their sap. 



(iii) A short account of recent experiments to determine by means of indicators 

 the pH of guard cells, &c, both in sections and entire leaves under various environ- 

 mental conditions, pointing to the conclusion that the pH and turgidity of the guard 

 cells varies readily with the C0 2 concentration of the leaf as a whole — regulated 

 normally by its photosynthetic v. its respiratory activity. 



Dr. Harold Wager, F.R.S. — The Effect of Light on Chlorophyll. 



Dr. F. W. Went. — Growth-promoting Substances and the Explanation of 

 Phototropism . 



Small blocks of agar, gelatine and silicate jelly, placed eccentrically upon the cut 

 surface of the coleoptile-stump of Avena, fail to induce curvature of the stump for the 

 first two hours. However, if the gel had been in contact with the freshly cut surface 

 of a coleoptile-top (in order to allow the growth-promoting substances to diffuse into 

 the gel), then the slumps will show a curvature such that the block is situated on the 

 convex side. It is possible to prove that the rate of curvature is determined by the 

 concentration of the growth-promoting substances. These substances appear to be 

 a Umiting factor for growth (in the sense of Blackman) up to a certain concentration, 

 above which concentration another limiting factor rather suddenly comes into play. 

 The latter factor I assume to be the amount of material used in the cell-extension, 

 1927 C C 



