272 Transactions. 



animals, &c., produce in time great changes, and are largely responsible 

 for tlie diverse plant forms now existing. And in the light of the present 

 knowledge as to the effect of environmental changes, it is impossible to con- 

 ceive that a change — huge, continuous, progressive — in a most vital condition, 

 extending for thousands or milhons of years, could be without a corre- 

 sponding change in the plant forms dependent upon it. 



The collection of the carbon essential to its existence is confined to the 

 surface of a plant. The surface is the part most directly concerned. 

 Hence, if any change occurs to balance this continuous impoverishment 

 of the air, it should be in the direction of a modification of the surface. 



Two distinct kinds of modification appear possible — (1) An increase in the 

 effectiveness of the original surface ; (2) an increase in the area of this surface. 



For our own present purpose the first may be neglected, as it seems 

 hardly possible to discover much about it. 



As to the second, palaeontoiogical records show that the earhest plants, 

 other than ferns, were practically leafless, and little branched. The ratio 

 of surface to mass was comparatively small. 



The lepidodendrons and calamites of the Devonian period were massive 

 plants showing quite a small surface. Later plants showed more branching, 

 more surface area ; but it was not until late in the Carboniferous era, by 

 which time immense amounts of CO 2 had been used up, that leaves were 

 common. 



An examination of plant-remains shows that since Carboniferous times 

 a fairly continuous increase in the ratio of surface to mass in the dominant 

 plants has taken place, until now the average flowering-plant shows an 

 abimdance of leaf, a ratio of surface to mass that has not been exceeded 

 at any previous period. 



It seems a fair inference, then, that the evolution of plant surface — in 

 other words, the evolution of the leaf — has been primarily due to the 

 necessity for more efficient CO 2 collection, and has been one of the results 

 of the progressive diminution of atmospheric CO 2. 



Arrhenius* points out that the actual amount of CO 2 in the air is so 

 small that in 1904 the coal burnt produced CO 2 equal to one-seventieth of 

 that of the air. The atmospheric CO., is consequently again increasing 

 or hkely to increase. 



One objection to the suggestion that decrease of CO 2 has developed 

 leaf surface is that ferns were amongst the earhest plants, and these exposed 

 a comparatively large surface from the begimiing. This was probably due 

 to an entirely different cause. 



Consider the factors in the development of plant surface. They appear 

 to be of two lands, respectively in favour of and against such development : 

 In favour — (1) Need of CO 2 ; (2) need of sunhght. Against — (1) Drought ; 

 (2) wind (mechanical efiect) ; (3) excessive sunUght. 



The early forests appear to have been dense, and then, as now, ferns 

 appear to have been forest-floor plants, hving in partial shade. Forest-floor 

 plants would be Ukely to have CO 2 in plenty, but little light. Light, however, 

 is absolutely essential, and a plant growing in the shade can increase its 

 light-collecting power only by spreading itself out as much as possible. 



While there are many drawbacks to a great extension of surface in an 

 exposed plant, on the forest-floor there is no wind to damage, no drought 

 to shrivel, no sun to scorch. 



* < 



' Worlds in the Making." 



