786 
The Action of Light on Plants . [December, 
extinction of their life if it be very intense, as that which 
comes direCtly from the sun. M. Macaire Prinseps observed 
that sheltering leaves from the aCtion of light prevents their 
change of colour in the autumn ; that if the entire leaf was 
placed in the dark, it fell off green ; if only a part, the rest 
of the parenchyma changed colour, and the covered portion 
retained its original colour. 
Mr. Hunt’s own experiments extended over seven years ; 
they were made at every season of the year, and the localities 
in which they were carried on were changed from the south- 
western extremity of the kingdom to the neighbourhood of 
the metropolis. The problem he wished to solve was the 
proportion and kind of influence exerted by light, heat, and 
aCtinism in the various stages of vegetable growth. The 
experiments were made under the aCtion of those radiations 
which had permeated variously coloured media, such as 
tinted glass and coloured transparent fluids. We should 
like, did space permit, to refer to the many precautions 
taken in order to render the experiments exaCb and con- 
clusive. We can, however, only give the results, referring 
our readers to Mr. Hunt’s own account for further particu- 
lars of his observations. These results are as follow ; — 
1. Light prevents the germination of seed. 
2. ACtinism quickens germination. 
3. Light aCts to effect the decomposition of carbonic acid 
by the growing plant. 
4. ACtinism and light are essential to the formation of the 
colouring-matter of leaves. 
5. Light and aCtinism, independent of the calorific rays, 
prevent the development of the reproductive organs 
of plants. 
6. The heat radiations corresponding with the extreme 
red rays of the speCtrum facilitate the flowering of 
plants and the perfecting of their reproductive prin- 
ciples. 
In order to ascertain the exaCt conditions in which the 
luminous aCtinic and calorific principles exist, Mr. Hunt 
made frequent examinations of the condition of the solar 
radiations. He found that in the spring the aCtinic principle 
is most aCtive, and as compared with light and heat it is 
considerably in excess. As the summer advances the quan- 
tity of light and heat increases relatively to the aCtinic 
principle in a very great degree. In the autumn light and 
aCtinism both diminish, and the calorific radiations are 
relatively to them by far the most extensive. 
