189 



favorable to vegetation, and the same may be said of high plateaus 

 where radiation is more powerful. 



7. The isanthesic lines, or lines of simultaneous flowering, do not 

 preserve any parallelism at different periods of the year; thus, the 

 line which shows where the lilac blooms on a given day of the month 

 passes ten days afterwards throuo;h another series of places where 

 the same phenomena is then occurring. 



Now. the zone comprised between these two lines has not the same 

 breadth throughout its whole extent, as would be the case with a zone 

 between two parallels of latitude. It is not even constant, since, for 

 example, a m(mth later the isanthesic lines will have quite different 

 forms, and localities that ^vere backward as compared with others 

 may then be in advance. 



8. The falling of the leaves is a phenomenon which in our climate 

 depends as nuich upon the current temperature as upon those which 

 have preceded. It is generally controlled by the first cold of autumn. 



Karl Fritsch (1881) gives the results of about ten years' observa- 

 tions of plants growing in the Botanical Garden at Vienna (1852- 

 1861). His list of plants embraced all those recorded in the previous 

 lists of Quetelet, Sendtner (1851), and his own, in all 1,600 species 

 and varieties, but of wdiich he has only used 889. The epochs ob- 

 served by him, as uniformly as possible throughout the ten years, 

 were the following : 



(1) The'first visibility of the upper surface of the leaf. 



(2) The complete development of the first flower, 



(3) The complete ripening of the first fruit. 



(4) The date at which a tree or bush has lost all of its foliage. 

 Having endeavored in vain to establish a connection between the 



moisture of the air and the growth of the plant, and finding it imprac- 

 ticable to take account of the moisture in the earth, Fritsch resolved 

 to reject observations made during special droughts or floods or otl^er 

 abnormal conditions and to consider only the sum of the average 

 daily temperatures. These mean daily temperatures he deduced from 

 the observations at 6 a. m. and 2 and 10 p. m., made at the Central 

 Meteorological Institution in Vienna, where the thermometer was 

 about 50 feet above the ground. The summation of the mean daily 

 temperatures for comparison with phenological phenomena counts 

 from the 1st of Januarj^ to the date of the observed epoch, and omits 

 all clays whose mean temperatures are 0° Reaumur or lower than that. 

 A comparison of the observations made on successive years on the same 

 plant shows that the time of blossoming is uncertain by only one or 

 two days in 96 per cent of all the plants, and the so-called " temi)era- 

 ture " or " thermal constant " is uncertain by 8 per cent of its amount 

 or less, in 97 per cent of all the plants. Similarly, for dates of ripen- 



