RELATIONS OF SOILS TO HEAT. 



309 



the soil, e. g., at night, dew must be deposited within the soil 

 down to the depth to which the nightly variation reaches, in- 

 creasing at that depth as the vapor from the warmer soil be- 

 low rises, to be in its turn condensed. There is thus formed 

 at that level a zone of greater moisture, which may sometimes 

 be noted in digging pits, by a deeper tint, without any cor- 

 responding variation in the nature of the soil. The daily re- 

 petition of this process, at varying depths, and its greater or 

 less recurrence at or near the limit-levels of monthly and even 

 annual variations, must exert a not inconsiderable influence 

 upon the vertical distribution of moisture in the soil ; which 

 instead of being usually found in horizontal bands or zones of 

 varying moisture-contents, is usually remarkably uniform for 

 considerable depths, despite the fitfully recurrent additions 

 from rains. It is at least probable that this process of dew- 

 formation within the soil materially assists capillarity in 

 effecting a measurably uniform vertical distribution of mois- 

 ture. ( See also page 207, chapter 1 1 ) . 



Plant-development under different Temperature Condi- 

 tions. In the arctic regions the ground, frozen in winter to 

 unknown depths, may thaw to only three to five feet during 

 the summer, notwithstanding the great length and continuous 

 sunshine of the arctic day. The shallow-rooted arctic flora 

 develops very rapidly under the influence of the continuous 

 daylight and heat, in the course of from five to eight weeks. 

 The seeds of these plants must, of course, be capable of ger- 

 minating at very low temperatures ; and as a matter of fact, 

 we find that both in the arctic regions and in the higher mount- 

 ains, certain plants are found growing and blooming on slopes 

 flecked with snow; each plant surrounded by a small circle 

 of bare ground, where the snow has been melted under the 

 influence of the dark-tinted earth and leaves. It is clear that 

 here germination has occurred, the foliage has been formed, 

 and the roots have been exercising their vegetable functions, 

 in ground soaked with water practically ice-cold. 



Germination of Seeds. \Yhile wild plants of special adapta- 

 tion may thrive in very low (or high) temperatures, it is also 

 true that few of our cultivated plants will germinate, and still 

 less grow thriftily, at such low temperatures. The limit be- 



