7^5 
i 879 »] Action of Light on Plants. 
milation of plants, a process which becomes manifest exter- 
nally by the exhalation of oxygen. Now, it is under the 
influence of light on the chlorophyll-containing cells that 
this evolution of oxygen is brought about. The recent ob- 
servations of Draper and Pfeffer have shown that in this 
adtion the solar spedtrum is not equally effedtive in all its 
parts ; that the yellow and least refrangible rays are those 
which adt with most intensity ; that the violet and other 
refrangible rays of the visible spedtrum take but a very 
subordinate part in assimilation ; and that the invisible rays 
which lie beyond the violet are wholly inoperative. 
In thus acknowledging the labours of Draper and Pfeffer, 
Dr. Allman forgot the previous researches of Mr. Robert 
Hunt, F.R.S., who investigated the subjedt forty years ago. 
Dr. Allman, however, in a letter to Mr. Hunt, observes, “ I 
have been refreshing my memory of your researches by reading 
your published account of them, and their completeness and 
conclusiveness render my regret the greater that I had not 
given them the recognition they so well merit.” The re- 
sults of Mr. Hunt’s “ Researches on the Influence of the 
Solar Rays on the Growth of Plants ” were described to the 
British Association in 1842 and 1846, and are given in his 
“ Researches on Light.”* In his history of the progress of 
the inquiry, he refers to Dr. Priestley’s experiments in 1779. 
These researches, which showed that carbonic acid was ab- 
sorbed by the plant, that under the influence of light it was 
decomposed, and that its oxygen was again liberated, were 
confirmed by the experiments of several naturalists and 
chemists, including Senebier, Ingenhousz, DeCandolle, 
Saussure, and Ritter. The general result of the investiga- 
tions on the chemistry of vegetation up to this point was, 
Dr. Hunt observes, that light was essential to healthful 
vegetation, but that the decomposition of the carbonic acid 
by the plant took place more decidedly under the influence 
of the most refrangible range of the spedtrum than of those 
which possessed superior illuminating power. In 1801 
Labillardiere communicated to the Philomathic Society his 
discovery that light was necessary to the development of 
pores in plants; and about the same time Vidlor Michellotti 
announced that light has a decided adtion on those germs 
which are exposed to it, — that this adtion is prejudicial to 
them, and it manifests its adtion by retarding their expan- 
sion if the light be weak or a defledted light, or by total 
* Researches on Light in its Chemical Relations, embracing a consideration 
of all the Photographic Processes. By Robert Hunt, F.R.S. Second Edition. 
London : Longmans. 1854. 
