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 fornuition 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 light has 

 but little connection with the temperature that promotes the further 

 action of the chlorophyl 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 decomj^osition 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 folloAvs: 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 Avith a certain 

 number of phanerogams in which the embryo is protected by thick 



