1901.] Some Beccnt Work on Diffusion. 547 



WEEKLY EVENING MEETING, 

 Friday, March 22, 1901. 



Sib James Crichtox-Browne, M.D. LL.D. F.R.S., Treasurer and 

 Vice-President, in the Chair. 



Horace T. Brown, Esq., LL.D. F.E.S. M.B.I. 



Some Recent Work on Diffusion. 



The subject of ray lecture, though essentially of a physical nature, 

 had its origin in what may be regarded as a no-man's land — a strip 

 of neutral territory which can be claimed exclusively neither by the 

 physicists nor the biologists. 



An attempt to reconcile some apparently contradictory facts con- 

 nected with the nutrition of plants has led, somewhat unexpectedly, 

 to an extension of the laws of gaseous diffusion, so that we shall 

 have to deal with one of those comparatively rare cases in which 

 biology has been able to react to some extent on physics. 



It has long been known that the primary source of the carbon of 

 all plants is the carbonic acid existing in small quantities in ordinary 

 atmospheric air, and that their green parts, more especially the 

 leaves, are able to utilise the energy of sunlight in decomposiug the 

 carbonic acid and water, and building up from their elements a whole 

 series of substances, such as sugars and starch, which contribute 

 directly to the nutrition of the plant. 



The immediate seat of this synthetic and assimilatory process is 

 found in the minute green chlorophyll granules, which occur in 

 great numbers within the cells of the leaf-tissue ; and one of the first 

 problems to be dealt with in the study of the process is to show in 

 what manner the highly dilute carbonic acid of the air can gain 

 entry into the leaf with sufficient rapidity to supply these assimilating 

 centres with material for the needs of the plant. 



In a typical leaf, such as is represented in section in the diagram, 

 both sides are covered with a cuticle and epidermis, pierced at regular 

 intervals on one or both sides with extremely minute openings, whose 

 size is capable of being regulated according to the requirements of 

 the plant. These are the stomates, which open out into a relatively 

 large cavity within the leaf, and this cavity in turn communicates 

 with the numerous and roomy air-spaces between the cells containing 

 the green chlorophyll granules. 



One of the most important functions of the stomates is un- 

 doubtedly to regulate the transpiration of water from the plant, but 



2 o 2 



