4 TRANSACTIONS AND PROCEEDINGS OF THE [Sess. lxi. 



The quantity of oxygen that is used up, and the corre- 

 sponding quantity of carbonic acid that is given out by 

 such a torpid creature as a plant during the night, is very 

 small, and is more than compensated by the reverse process 

 which takes place during half an hour's sunshine in the 

 morning. Although it is about a hundred years since the 

 main fact concerning the process of assimilation was known 

 — what may be called the upshot of the process — we are 

 yet very far from knowing how it is that plants take their 

 carbon from carbonic acid gas and convert it into their 

 own tissues. We can see the formation of starch in the 

 chlorophyll cells during sunshine, and its disappearance 

 (luring darkness, but as yet we know nothing certain 

 regarding the steps which lead up to the formation of 

 starch. Whether the chlorophyll takes a formative part 

 in the making of starch, or whether it simply acts as a 

 screen to allow only select rays of light to reach the 

 laboratory where the carbonic acid is being decomposed, 

 and whether this pioduct is hypochlorin, as Pringsheim 

 suggests, or formic aldehyde as some have supposed, we 

 know nothing sure. The chemistry of the carbohydrates is 

 a very intricate subject, and difficult of exploration, and 

 only the rudiments of it are as yet known. 



If that is the case with the carbohydrates, it is so in a 

 .still more marked degree with the nitrogenous constituents 

 of plants. 



The molecule of starch is simplicity itself compared with 

 the molecule of albumen, which may be regarded as the 

 finished article of the nitrogenous kind that is built up in 

 the tissues of plants. It has been estimated that the 

 molecule of albumen may consist of from 3000 to 5000 

 atoms. Such estimates are mere guesses, scarce worth 

 considering, but they serve the purpose of impressing 

 upon tlie mind the extreme complexity of some of the 

 nitrogenous su])stances of which plants are composed, and 

 the enormous difficulties which that complexity places in 

 the way of their investigation. 



It is to the nitrogenous parts of plants, and especially 

 the nitrogenous food of plants, that 1 wish to direct your 

 attention for a short time; and I have been prompted to do 

 .so from the knowledge that there will be brouizht before 



