1003.] The Cosmical Function of the Green Plant. 459 



available in the process of assimilation, we shall readily see that the 

 difference of these areas for the horizontal and for the vertical leaf 

 will not be considerable, whilst in the latter case the injurious effect 

 of a great excess of radiant energy at high sun, when the saturation 

 of the air is at its minimum, and the evaporation consequently at its 

 maximum, will be spared to the plant. For lower latitudes the result 

 will be still more favourable. 



The same considerations hold good when applied to leaves having a 

 woolly appearance on their upper surface, which may be compared to 

 a plate of ground glass. Provided the scattering of light does not 

 diminish its intensity to less than one-half at noon, the plant will be 

 only the gainer in respect of a more economicale vaporation, without 

 any injury to its nutrition. 



Here I must put a stop to this analysis of the consequences that may 

 be drawn from the study of the optical properties of chlorophyll. It 

 seems to me that all that we know about the function of chlorophyll 

 may be deduced from its optical properties, and this result is quite 

 natural, since the process of assimilation of carbon is at the same time 

 the process of assimilation of sunlight.* The chlorophyll function may 

 l)e thus considered as the cosmical function of the plant. Professor 

 Boltzman in a remarkable address, read before the Academy of Vienna 

 in 1886, expressed this idea in the following eloquent passage: " Der 

 allgemeine Daseinskampf der Lebenswesen ist daher nicht ein Kampf 

 um die Grundstoffe die Grundstoffe aller Organismen sind in Luft, 

 Wasser und Erdboden im Ueberflusse vorhanden auch nicht um 

 Energie, welche in Form von Warme, leider unverwandelbar, in jedem 

 Korper reichlich enthalten ist, sondern ein Kampf um die Entropie, 

 welche durch den Uebergang der Energie von der heissen Sonne zur 

 kalten Erde disponibel wird. Diesen Uebergang moglichst auszunutzen,. 

 breiten die Pflanzen die imermessliche Flache ihrer Blatter aus, und 

 zwingen die Sonnenenergie, in noch unerforschter Weise, ehe sie auf 

 das Temperaturniveau der Erdobernache herabsinken, chemishe 

 Synthesen auszufiihren, von denen man in unseren Laboratorien noch 

 keine Ahnung hat. Die Producte dieser chemischen Kiiche bilden das 

 Kampf object fiir die Thierwelt." From this point of view this field of 

 inquiry on the cosmical function of the green plant is a borderland of 

 two of the greatest generalisations of last century, associated with the 

 names of Lord Kelvin and Charles Darwin the principle of dissipation 

 of energy and the principle of the struggle for life. And it is with a 

 feeling of veneration that I recollect these words heard many years 

 ago, at Down, from the late Mr. Darwin himself: "Chlorophyll is 

 perhaps the most interesting of organic substances." 



* " The Assimilation of Light by the Plant" was the original title of my Russian, 

 paper on the subject ; quite lately I was glid to see this expression employed by 

 Profess Dr Ostwald in his ' Xaturpliilosophie.' 



