THE 



POPULAR SCIENCE 



MONTHLY 



MAY, 1914 



THE MEASUREMENT OF ENVIRONIC FACTORS AND 

 THEIR BIOLOGIC EFFECTS 1 



By Dr. D. T. MacDOUGAL 



DIRECTOR, DEPARTMENT OF BOTANICAL RESEARCH, CARNEGIE INSTITUTION OF WASHINGTON 



THE simpler forms of plants in earlier geologic times lived in swamps 

 and along seashores and under the equable conditions furnished 

 did not attain anything beyond a primitive and elementary development. 

 The chief bar to escape from the restricted moist habitats consisted in 

 the fact that the life cycle of the plant included alternating generations 

 in one of which, the sexual generation or gametophyte, reproduction was 

 possible only in the presence of water. Finally, however, the spore 

 which gave rise to this sexual generation began to germinate in place on 

 the other generation and the resulting gametophyte was produced, and 

 remained enclosed in the tissues of the sporophyte as it is among the 

 seed-plants of the present day. 



The domination of the sporophyte in this manner vastly increased 

 the possibilities of evolutionary development, and when this plastic self- 

 contained type of plant began to move out over the broad spaces of the 

 world, all the ranges of temperature afforded by the earth's surface, as 

 well as of moisture, illumination, concentration of the solutions in the 

 soils, alkalinity, etc., were encountered to which to-day the manifold 

 types of plants stand in a delicate adjustment. 



Water was the chief determining factor when the vegetal organism 

 was in a separated-generation stage, and it continued to be the most 

 potent agency in evolution and differentiation as the new combined 

 individual moved away from the swamps arid shores to the occupation 

 of the drier slopes of valleys and mountains and finally into the most 

 arid of deserts. 



i Formal abstract of lecture given before the trustees of the Carnegie Insti- 

 tution of Washington, in connection with the Annual Meeting, December 1, 

 1913. 



VOL. LXXXIV. — 29. 



