510 EXPERIMENT STATION RECORD. 



set in <»ii a date varying from year to year, the date de[H'nding primarily 

 on tlie climate of each year, yet to reach this iiliasse the plant requires 

 an amount of heat that is constant from year to year. A plant may, 

 therefore, be considered as a means for measuring lieat, and the begin- 

 ning of a certain vegetal phase is also a stanchird lor measuring a cei- 

 tain sum total of lieat supplied up to that date, and this sum total 

 expresses the measure of heat required by the plant to reach the phase 

 in (juestion. 



Ditferent sums were, and must be, obtained with thcrnu^meters of 

 dillerent construction, but not so with the same instruments. Most 

 investigators who have occupied themselves with the problem of ther- 

 mal constants have, in their calculations, made exclusive use of shade 

 temperatures, either the mean or the maximum temperatures; and this 

 is still the case at Russian stations. But all methods of paralleling 

 vegetal development with thermometric values are open to numerous 

 and legitimate objections. Above all, stress must be laid on the fact 

 that plant life, in its period of growth, is governed by the joint effect 

 of the chmatic factors, heat, light, and moisture, but that Ave are 

 hardly able to reciprocally weigh their individual i)ower. Further, 

 that which causes our trees and shrubs to bud and blossom in the 

 spring is not only a consequence of the gradual increase of heat, 

 together with light and snflficieut moisture, but these vegetal phe- 

 nomena are also the effects of an obscure biological property, a certain 

 interior rythm of the ligneous plants, by which they perform annually, 

 with suitable periods of rest, the same functions in regular sequence. 

 This rythm has adjusted itself to the average climate. Connected with 

 this is the fact tiiat the duration of the period of rest also exerts an 

 influence on the development of vegetation, and, therefore, can not 

 arbitrarily be shortened so as always to produce by an artificial increase 

 of temperature (liot liouses), in a relatively short time, the same effect 

 as that produced by a lower temperature continued through a longer 

 l)eriod. 



This reckoning must, without doubt, begin from a natural zero point 

 of vegetation. But where is this point in nature? With deciduous 

 plants we may safely select the day of sowing similarly treated kinds 

 of seeds, but with our ligneous plants it should probably be the begin- 

 ning of the period of rest or the beginning of vegetal activity. But 

 the beginning of either period is very hard to determine, for pheno- 

 logic observations have demonstrated that the vegetal activity of 

 buds does not entirely cease even during the winter period of rest- 

 Tlie first day of January, which has been selected in many methods as 

 a day of complete winter rest, is therefore a somewhat arbitrary date, 

 although the error is propably a small one. Drude suggests the date 

 of tlie winter solstice, or the first day of December. Ziegler (Frank- 

 fort-on-the-]\Iain), who otherwise follows Hotrmann's method, reckons 

 from the beginning of a vegetal phase in one year to the beginning 

 of th(! same phase in the year following. 



