179 



a be. a coefficient that defines the rate of development so that tlic 

 reciprocal of a defines the longevity of the plant; 



n be a coefficient that defines the sensitivene.ss of the plant to tem- 

 perature, so that as n increases a given change in x has a less effect 

 on the rate of growth and therefore the plant can flourish in a wider 

 range of temperature; therefore its geographical distribution may 

 be wider, hence Coutagne calls n a coefficient of ubiquity ; 



c be the temperature at Avhich the most rapid development is possi- 

 ble under the most favorable conditions of growth or the temperature 

 optinnnn ; plants with a large value of c must live nearer the equator 

 than those having small values of r-; therefore c is called the index 

 of tropica lity. 



According to Coutagne these quantities are bound together by the 

 formula : 



v=a e 

 This formula represents the momentary rate of development, so 

 that the total duration of the growth^is to be found by integrating 

 this expression, which result is written as follows : 



'^Sr\ 



ax 



Van Tieghem, like Coutagne and others, finds that for each special 

 phase of vegetation, germination, heading, flowering, or ripening, and 

 for each age of a perennial plant there exists a special relation 

 between the temperature, the light, the moisture, and the chemical 

 composition of the soil and water that is most favorable to growth. 

 We have, therefore, to decide whether the same formula of develop- 

 ment can represent the growth in each of these phases as well as 

 throughout the whole career of the plant. As we have before said, the 

 plant can only rearrange the inorganic products that it receives and 

 develop its own structure by utilizing the molecular energy contained 

 in the sunshine or some equivalent light. Its growth does not depend 

 upon any force contained within the plant nor on the temperature, as 

 such, but on the quality of the radiation ; therefore any formula that 

 considers temperature only must be a very imperfect presentation 

 of the growth, especially in those stages subsequent to the full develop- 

 ment of the leaf and flower. 



Lippincott (1863, p. 506) gives a few items relative to the phenol- 

 ogy' of wheat in America and the origin of the varieties known as 

 Lambert's Mediterranean China (or Black Tea), Hunter's, Fenton, 

 Piper's, which were all due to judicious selection and careful culture. 



The average wheat crop of England is stated to be 36 bushels per 

 acre and that of the United States 15 or less, which large difference 

 is, he thinks, the result of judicious cultivation and care in the choice 



