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SCIENCE 



[N. S. Vol. LIII. No. 1369 



manui-e," meanmg that tte same end results 

 could thus be obtained. Why? There is a 

 bewildering array of hypotheses as to why 

 tillage tends to increase yields, including 

 better mechanical conditions; improved aera- 

 tion; increased nitrification; additional car- 

 bon dioxide; mixing of the soil; elimination 

 of weeds; and in dry regions particularly con- 

 servation of moisture. There may be and 

 probably is some truth in all of these ex- 

 planations but exact data on any of them 

 are far from abundant. The really definite 

 knowledge is empirical, namely, that tillage 

 methods do tend to increase yields. 



The breeding of plants has been a most 

 potent factor in securing larger and better 

 yields. Our knowledge of genetic phenomena 

 has been enormously increased in recent 

 years from the activity incited by the redis- 

 covery of Mendel's law. The effect of this 

 greatly increased knowledge of genetics has 

 inspired many immoderate statements as to 

 its effect on agriculture. Thus one writer 

 says " Through scientific work in the study of 

 heredity, we have learned to multiply the 

 races of our useful plants so that they may 

 fit in more exactly to the variable conditions 

 in which plants must be grown," and that 

 Mendel's law " is the basis of most of our 

 work in the study of heredity and this in 

 turn has made agriculture scientific." It is 

 pleasing to learn that a bit of leaven like 

 tliis is able to uplift all agriculture into the 

 condition called scientific, while presumably 

 it was before something different. As a 

 matter of fact, the practical value of Men- 

 delian knowledge to plant breeding is dis- 

 appointingly small. Witness the innumerable 

 improved varieties in all our cultivated plants 

 long antedating Mendel. Consider the lilies, 

 the roses, the chrysanthemums, the carnations, 

 the tulips, indeed, any plant much cultivated, 

 and ponder upon the infinite amount of work 

 that led to their development — all without the 

 guidance of any scientific theory. This ad- 

 mission does not discount the tremendous 

 value of the new knowledge of genetics which 

 gives us so great an insight into the factors 

 involved in plant variations. 



The nature of plant diseases and the meth- 

 ods discovered for their control is a contribu- 

 tion to plant culture for which the botanists 

 of the schools may rightly claim large credit. 

 This is clear in spite of the fact that farmers 

 and gardeners had before the day of plant 

 pathologists found out the efficacy of blue- 

 stone for wheat smut and sulphur for mil- 

 dew; and against other diseases had developed 

 resistant or immune varieties. The develop- 

 ment of phytopathology is an index, I believe, 

 of what might well happen in other fields of 

 plant culture, if trained botanical workers will 

 wholeheartedly engage in its problems and 

 avoid being attracted more to the purely 

 scientific problems than to those of cultural 

 import. 



The climatic complex of factors is difficult 

 to evaluate. IN'umerous attempts have been 

 made to correlate growth and yield with the 

 curves of temperature and of moisture pre- 

 cipitation and even specifically to outline the 

 limits of the future extension of wheat cul- 

 ture northward. Thus far these attempts 

 have not thrown any great light on the prob- 

 lems of climatic adaptations. 



I must not omit, however, the recent illumi- 

 nating contribution of Garner and Allard, 

 who have discovered the remarkable reactions 

 of plants to tlie length of daily illumination. 

 Any one who has cultivated plants has come 

 to realize the extraordinary way in which they 

 behave under different conditions, one might 

 say the vagaries which they exhibit. One of 

 these is the manner in which most plants 

 speed up their maturing in fall. The farmer 

 says the plant is hurrying to get ripe before 

 frost. Several vague theories were current 

 among plant culturists as to the cause of this 

 phenomenon, one that the stimulating factor 

 was the increasing difference between day and 

 night temperatures, another that it was due to 

 the increased temperature of the soil. It is 

 remarkable to how high a degree the tempera- 

 ture factor was assumed in every periodic 

 phenomenon. Garner and Allard have ac- 

 cimaulated a mass of experimental data that 

 leave no room for doubt that the stimulating 

 factor is associated with the daily length of 



