TRANSACTIONS OF SECTION K. 925 



of Cormophyta into Acrotrya, Amphibrya, and Acramphibrya, proposed by Unt^er 

 and Endhcher. Still it cannot be denied that anatomical characters have been 

 found useful, if not absolutely conclusive, in sug-gesting' affinities, especially in the 

 determination of fossil remains. A large proportion of our tnowledge of extinct 

 plants, to which I have already alluded, is based solely upon the anatomical 

 structure of the vegetative organs ; and although affinities inferred from such 

 evidence cannot be regarded as final, they suffice for a provisional classification 

 until they are confirmed or disproved by the discovery and investigation of the 

 reproductive organs. 



Physiology. 



The last branch of botanical science which I propose to pass in review is that 

 of physiology. We may well begin with the nutritive processes. At the close of 

 the eighteenth century there was practically no coherent theory of nutrition ; such 

 as it was it amounted to little more than the conclusion arrived at by van He'lmont 

 a century and a half earlier, that plants require only water for their food, and are 

 able to form from it all the different constituents of their bodies. It is true that the 

 important discovery had been made ai)d pursued by Priestley (1772), Ingen-Housz 

 (1780), and Senebier (1782) that gTeen plants exposed to light absorb carb'ou dioxide 

 and evolve free oxygen ; but this gaseous interchange had not been shown to be the 

 expression of a nutritive process. At the opening of the nineteenth century (1804) 

 this connection was established by de Saussure, in his classical ' Recherches 

 Chimiques,' who demonstrated that, whilst absorbing carbon dioxide and evolving 

 oxygen, green plants gain in dry weight ; and he further contributed to the eluci- 

 dation of the problem of nutrition by showing that, whilst assimilating carbon 

 dioxide, green plants also assimilate the hydrogen and oxygen of water. 



Three questions naturally arose in connection with de" Saussure's statement of 

 the case : What is the nature of the organic substance formed ? What is the 

 function of the_ chlorophyll ? What is the part played by light ? It was far on in 

 the ceutury before answers were forthcoming. 



With regard to the first of these questions the researches of Boussingault 

 (1864) and others established the fact that the volume of carbon dioxide absorbed 

 and that of oxygen evolved in connection with the process are approximately equal. 

 Further, the frequent presence of starch in the chloroplastids, to which Mohl first 

 drew attention (1837), was subsequently found by Sachs (1802) to be closely con- 

 nected with the assimilation of carbon dioxide. The conclusion drawn from these 

 facts is that the gain in dry weight accompanying the assimilation of carbon 

 dioxide is due to the formation, in the first instance, of organic substance havino- 

 the composition of a carbohydrate ; a conclusion which may be expressed by the 

 equation 



C0„ + ILO = CILO + Oo. 



The questions with regard to chlorophyll and to light are so intimately con- 

 nected that they must be considered together. The first step towards their 

 solution was the investigation of the relative activity of light of different colours 

 ori<,'inally undertaken by Senebier (1782) and subsequently repeated by Daubeny 

 (LS-ib), with the result that red and orange light was found to promote assimila- 

 tion in a higher degree than blue or violet light. Shortly afterwards Draper (1843) 

 espe^im^ntiug with an actual solar spectrum, concluded that the most active rays 

 are the orange and yellow; a conclusion which was generally accepted for many 

 years. But in the meantime the properties of the green colouring matter of plants 

 (^to which Pelletier and Caveutou gave the name ' chloropliyll ' in 1817) were bein"- 

 investigated. Brewster discovered in 1834 that an alcoholic extract of o-reeu 

 leaves presents a characteristic absorption spectrum ; but many years elapsed before 

 any attempt was made to connect this property with the pliysi'ological activity of 

 chlorophyll. It was not until 1871-72 that Lommel and N. J. C. Miiller pointed 

 out that the rays of the spectrum which are most completely absorbed b\' chloro- 

 phyll are just those which are most efficient in the assimilation of carbon dioxide. 

 Subsequent researches, particularly those of Timiriazefl' (1877), and those of En»el- 



