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AMERICAN JOURNAL OF BOTANY 
[Vol. 8, 
Crop Physiology 
At the present time, progress in crop improvement is waiting on a fuller 
knowledge of crop physiology. To the farmer, as to the agronomist, the 
value of crop varieties is measured in terms of their performance in pounds or 
bushels. We know by experimentation that one variety of any given crop 
yields better under a certain set of conditions than do other varieties of 
that crop, while under different conditions this same variety may be com- 
paratively unproductive. We know that crops vary greatly in their com- 
parative resistance to disease, to cold, to frost, to heat, to drought, to soil 
alkali, and to all the other unfavorable factors in the environment. In 
the same way, some varieties seem unable to stand prosperity. Given what 
apparently are very favorable conditions, they seem unable to make a 
proportionate response in production. These things we know, but what we 
do not yet know is why these things are so. That is the next and most 
immediate problem in crop improvement. 
In the practice of medicine, the detailed study of the functioning of 
the various members of the human body has been held indispensable to 
a proper diagnosis of diseased conditions. In the case of even a single 
one of our most important crop plants, however, no such detailed study 
has been made. We attempt to acclimatize them in various parts of the 
world, to make them productive under a wide range of climatic conditions, 
and to breed them to produce forms with very specialized adaptations, 
without this fundamental knowledge of their relations, derived from ade- 
quate research. 
The relative and actual importance of such external factors as light, 
air temperature, humidity, soil temperature, soil texture, and soil solution 
in their effect on the growing crop plant at different stages of growth, from 
germination to maturity, are very imperfectly known quantities today. 
Without doubt, the increasing determination of the values of these and other 
factors will have a profound influence on the practices of crop production 
and ultimately on the quantity and quality of the product. 
The recent discovery by Garner and Allard, of the effect on plant 
growth caused by varying the duration of the daily light period, not only 
is shaking the foundation of our theories and opens leads toward many 
unsolved problems, but is highly suggestive of results that will be obtained 
when other and equally fundamental researches are made in the realm of 
crop physiology. Some of the lines along which such research should be 
directed have been mentioned. Next to light, the fundamental factors 
are temperature, water, and food. 
Several unrelated studies of temperature relations have been made, 
but research to date has touched only the fringe of this problem. Tem- 
perature studies are vital to such problems as crop adaptations, including 
the extension of the areas of fall-sown crops, as against spring-sown; the 
comparative development of root and shoot and the speed of development 
