386 SECTIONAL TRANSACTIONS.— K. 



the concentration of which increases from the base towards the top of the coleoptile. 

 This gives a plausible explanation for the specific distribution of the growth-rate over 

 the coleoptile. 



Experiments by Dolk (as yet unpublished) show that the absence of growth- ' 

 promoting substances in the coleoptile of Avena causes an almost absolute stagnation 

 in its growth. It follows, therefore, that any curvature must be caused by a one- 

 sided increase or decrease of the growth-promoting substances. From this point 

 of view I have analysed the problem of phototropic curvature ; the results already 

 obtained are in good agreement with the above assumptions. 



Dr. W. H. Pearsall. — Metabolic Effects of Nitrogen. 



Dr. James Ewing and Miss E. Rotjghton.— The Influence of Hydrogen-ion 

 Concentration on the Swelling of Plant Tissues. 



It has been found by several observers that plant protoplasm in its physico- 

 chemical reactions resembles protein in showing marked amphoteric properties. In 

 some respects plant tissues behave in similar fashion. The differential swelling 

 which potato or beet tissue exhibits when placed in buffered solutions over a range 

 of hydrogen-ion concentration from pH 2 to pH 7 cannot be wholly explained by osmotic 

 phenomena. Wliile a large part of the swelling is undoubtedly due to osmosis, it is 

 considered that the differences in water absorption at different pH value may be due, 

 either to differences in hydration of the protoplasm or constituent proteins, or to 

 some changed condition in the permeability of the cell membrane. 



Tuesday, September 6. 



Joint Discussion with Section A (Cosmical Physics Department) and 

 Section C on The Climates of the Past. 



Prof. A. C. Seward, F.R.S.— 



Few problems make so strong an appeal to the imagination as those relating to 

 climatic retrospects. It is important, in the first place, to state the nature of the 

 evidence bearing on the problem ; to consider whether or not the evidence is such as 

 to bear the conclusions drawn from it. Fossil plants from different geological forma- 

 tions are compared with recent genera or species ; it is generally assumed that the 

 conditions under which the existing plants grow may be accepted as criteria of climatic 

 conditions in the past. As examples of fossil floras, which appear to afford evidence 

 of much higher temperatures in Arctic regions than at present, the following are 

 selected for brief description : Upper Devonian ; Rheetic ; Cretaceous. 



It is suggested that too little account has been taken of (I) the fact that closely 

 allied plants have different reactions to climate, or of (II) the possibility of specifically 

 identical plants becoming modified in their power of resistance during lapse of time 

 as senility replaces juvenile vigour. In this connection reference may be made to 

 recent work by Fernald on the persistence of plants in the Gaspe Peninsula during 

 the last glacial period. There is the further consideration, that certain genera 

 (e.g. Gleichenia), now mainly tropical in their distribution, are represented by some 

 species which flourish at high altitudes where conditions are much less genial. 



In the second place, having formed an estimate of the climatic contrasts between 

 the past and the present, our aim is to discover the most probable explanation of 

 these differences. Does the Wegener hypothesis offer a satisfactory solution ? Can 

 we, by altering the distribution of land and water, so far as is consistent with geological 

 data, with its concomitant effects on oceanic and atmospheric circulation, reproduce 

 temperatures believed to be required on paleeobotanical grounds, without having 

 recourse to Wegener's floating continents and shifting poles ? 



Dr. G. C. Simpson, F.R.S., Mr. C. E. P. Brooks, Dr. D. H. Scott, F.R.S., 

 Prof. J. W. Gregory, F.R.S., Dr. H. Hamshaw Thomas, Mr. J. 

 Walton, Mr. R. J. Matthews, Mr. R. D'O. Good. 



