OCTOBEB 1, 1909] 



SCIENCE 



445 



agricultural research and educational institu- 

 tions. They form a good key to the body of 

 knowledge already accumulated. They are in 

 part history. Their substance gives prophecy 

 of the greater things which are to come. 



In another decade or two science will have 

 not only doubled our definite knowledge of 

 things agricultural, but will have reduced this 

 body of thought to pedagogic form and will 

 have secured it a place beside the three R's 

 in our rural schools. Had these volumes been 

 written two or three years later the author 

 would have placed the consolidated rural 

 school — the farm school out in the open coun- 

 try or in the village — foremost as an educa- 

 tional agency in country life. And the pub- 

 lishers will find that these rapidly multiplying 

 schools, so organized as to support school and 

 circulating libraries in the rural communities, 

 will be one of their largest markets for sets of 

 these volumes. 



This encyclopedia will prove of especial 

 value in the library of all secondary schools 

 and colleges, whether patronized by city- or 

 country-bred youth. It will be a source of 

 information not only in regard to subject 

 matter for use in class work — but as the basis 

 of essays, debates and other literary efforts. 

 The presence of this body of knowledge will 

 make it possible for teachers to assign more 

 written work on concrete subjects, that the 

 pupils may devote the actual composition to 

 writing facts, rather than to trying to dig up 

 abstruse thoughts which do not exist in their 

 minds. The opportunity afforded for our 

 youth to know more not only of various as- 

 I)ect8 of outdoor life, but of our greatest in- 

 dustry and of our most numerous industrial 

 class is important. Not only is it of advan- 

 tage for city youth to have clear conceptions 

 of farms and farmers, but it is important that 

 country youth should know more of other 

 farms and of the farmers of other communities. 

 W. M. Hays 



U. S. Depabtmext of Aoeicultcbe 



Zur Biologie des Chlorophylls. Lauhfarhe 

 und HimmeUlicht. Vergelhung und Eliole- 

 ment. Von E. Stahl, Professor in Jena. 

 Jena, 1909. 



Professor Stahl is one of the foremost bot- 

 anists of that school of biologists which at- 

 tempts to interpret the facts of nature on the 

 hypothesis that everything which endures is 

 useful, that the qualities of an organism 

 which are useless or harmful presently disap- 

 pear or cause the organism to disappear in 

 the struggle for existence. In the present con- 

 tribution to philosophical biology Stahl has 

 selected for consideration a subject of prime 

 importance: Why are plants green, why are 

 the organs in which plants manufacture food 

 from inorganic materials green? 



It is remarkable that the plants of the 

 earth's surface are green or have green leaves. 

 Few land plants are otherwise colored; the 

 plants living below low-tide mark are, gener- 

 ally speaking, red; many plants between the 

 tide marks, or close to the surface of the sea, 

 are more or less olive, that is, greenish-brown; 

 a large group of more or less unicellular algie, 

 living on damp soil or in shallow water, both 

 fresh and salt, are olive-brown. Certain bac- 

 teria, constituting a small group, are purplish- 

 red. Green i.s, then, the predominant color of 

 the vegetation of land and sea. And one is 

 disposed to believe that this has always been 

 the case since plants came into existence. 



The manufacture of food by plants depends 

 upon energy acquired by absorbing light. 

 The materials first used in manufacturing 

 food are water, of which there is a more or 

 less abundant supply wherever plants exist, 

 and carbon dioxide, of which there has been 

 for ages only a very meager proportion in the 

 atmosphere. The supply of carbon dioxide is 

 practically constant, the supply of water vari- 

 able both locally and generally, the supply of 

 energy, of light, varies daily, varies locally. 

 The process of food manufacture depends, 

 then, upon two variables and one constant, 

 but the constant is very small in proportion 

 to the other ingredients of the air. I wish to 

 emphasize the small proportion of COj in the 

 air, for unless one realizes this, one will fail 

 to understand why plants, by absorbing more 

 of the available solar energy, could not im- 

 prove their present case. In fact, one criti- 



