ley to 98 days. In the same geographical latitude, a longer vegetation period 

 is found in the west than in the east. 



'Jhe causes of the shortening of the growth periods, therefore, cannot 

 lie in the warmth which the plants receive at a corresponding degree of lati- 

 tude, for otherwise the plants in the south would have passed through their 

 development considerably more quickly than in the north, also since the 

 southern black soil is raised to a higher temperature than the heavier, often 

 clayey and damp soil of the north. l)esides this, the lack of moisture in the 

 south hastens maturity very greatly. Some other factor must therefore be 

 determinative. Kowalewski states this to be ihe lerujth of the insolation. 

 He now assumes May 5th to be the mean time for sowing oats and August 

 20th as the mean time for harvesting them, finding thereby an insolation 

 period of 2000 hours for the 98 days of vegetation in Archangel. If the 

 period of bright nights be added to this, there is an increase amounting to 

 2240 hours. Kherson oats are sown on March 20, harvested on July 20th. 

 In this 123 days of vegetation, however, only 1850 insolation hours obtain. 

 Further, as Kowalewski says, it must be noted that the cultivated species of 

 the north are adapted to a lesser degree of warmth. Therefore, when 

 brought to the south, they ripen comparatively earlier. This result agrees 

 with the one found by Schiibeler' which will be mentioned later. Similar 

 observations are said to have been made in Canada also. 



In further explanation of the change in the length of vegetation, Kowa- 

 lewski brings forward the greater intensity of illumination, the small cloud 

 masses and the greater humidity of the atmosphere and, supported by Fa- 

 mintzin's investigations, he believes, for example, that the light optimum 

 for assimilation is exceeded in the south and therefore has a retarding in- 

 fluence. This would correspond to the yellowing of the shade-loving plants, 

 when grown in high mountains. It is not necessary to fall back upon the 

 theory of the retarding action of the southern excess of light, if Wiesner's 

 theory be accepted. In explaining the utilization of light on the part of 

 plants in the far north, Wiesner- emphasizes, according to his investigations, 

 the fact that in regions of the far north (Tromso), with an equal elevation 

 of the sun and an equal clouding of the sky, the chemical intensity of the 

 daylight has been shown to be greater than in Vienna and Cairo, but less 

 than in Buitenzorg in Java. The light factor of the far northern regions is dis- 

 tinguished in its illuminating quality by a relatively marked equability which 

 obtains in no other locality where plants flourish. The plants of the arctic 

 vegetative zone receive the greatest amount of light as a whole. Here, in 

 the low growing plants there is no self-shading due to their own foliage, 

 and even woody plants in adjacent southern regions show only a minimum 

 amount of shade-producing branches. 



1 Schiibeler, Die Pflunzenv.elt Norwegens. 



- Wiesnor, .1., Beitrage zur Kenntnis des photo-chemischen Klimas im arktis- 

 chen Gebiete. Sitz. Akad. d. Wiss. Wien CVII, cit. Bot. Jahresb. 1898, I, p. r.86. 



