38 
light should be cut off both the plants having chlorophyl, and also the 
plants that do not have it, would disappear from the surface of the 
globe. . 
Berthelot, in his essay on the mechanics of chemistry as based on 
thermochemistry, shows that the action of the light is demonstrated 
by the formation of complex chemical effects, isomeric changes, and 
more complex reactions. For instance, the combination of free oxygen 
is stimulated in a great many cases by the action of light, as is shown 
by the bleaching of fabrics of any kind exposed to the air and by the 
oxidation of volatile oils. All the oxidizing in reactions brought 
about by the action of light is exothermic—that is to say, there is a 
loss of energy in the transition from the compound body to its ele- 
mentary components and a disengagement of heat. The light plays 
the role of a determining agent. On the other hand, when a complex 
body is built up in the cells of a plant, by drawing in elementary 
bodies from the atmosphere and soil, the reaction is endothermic, and 
solar heat is absorbed and rendered latent in the plant. 
Sachs, Wiesner, and Mikosh would seem to have established the 
principle that the formation of the green matter of a plant is not 
dependent wholly on the light as such, but also demands a certain 
temperature, varying between 0° and 35° C., for the various plants of 
Europe. They show also that an increase in the temperature of the 
atmosphere, with equal increase of light, increases the rapidity of 
the formation of the chlorophyl up to a certain maximum tempera- 
ture, and that in proportion as the temperature departs from this 
favorable maximum, either above or below, the formation of the green 
matter becomes less and less active, until when the limits 0° or 35° C. 
are exceeded it ceases altogether. But the temperature most favor- 
able for the formation of chlorophyl under the action of hght has 
but little connection with the temperature that promotes the further 
action of the chlorophy! after it has been formed within the plant. 
Thus Timiriazeff (1880) shows that the activity of the chlorophyl 
consists in the absorption of certain radiations; but in order that these 
radiations may act it does not suffice merely that they should be 
absorbed; it is further necessary that there should be a very consid- 
erable intensity of heat, in order to furnish to the chlorophyl the 
definite number of calories necessary for the decomposition of the 
carbonic-acid gas taken in from the atmosphere. 
In general, under ordinary conditions light is indispensable to the 
formation of chlorophyl. To this general law there are a few appar- 
ent exceptions, as follows: The embryos of the genera Pinus and 
Thuya have their cotyledons colored an intense green at the moment 
of germination, even when they have been or appear to have been 
completely deprived of the action of light. So also with a certain 
number of phanerogams in which the embryo is protected by thick 
