CLIMATIC FACTORS 125 



3. The Pinetum montanae rhodorosum. Snow cover for 5^2 to 6 months 

 mostly on steep warm slopes. Rhododendron hirsutum is dominant. The mosses 

 Ilylocomiutn triquelrum and Ilypnum schreberi in dryer places, //. splendens in 

 moister places, and the shrubby Erica carnea are abundant. 



4. The Pinetum montanae ericosum. Snow cover similar but on more level 

 ground. Erica carnea and Vacciniuin Hits idaea are dominant. 



Obviously, in this distribution the duration of snow cover is not the 

 only factor, but it is one of the most influential. 



B. Humidity 



The humidity of the air regulates the loss of water by transpiration; 

 the dryer the air the greater the water loss. All conditions of the 

 habitat which reduce humidity, such as high temperature of the air, 

 intense radiation, strong winds, are accompanied by an increase in the 

 loss of water. 



Gordjagin (1930) has shown that the loss of water from the leaves 

 of conifers and evergreen oaks continues even during the coldest 

 weather and that toward the end of the winter the losses by transpira- 

 tion of Quercus pedunculata are compensated by the water drawn from 

 the oldest parts of the tree. 



Hygromorphy, Xeromorphy.— Humidity of the air directly influ- 

 ences the form and structure of plant organs. Eberhardt's (1903) 

 experiments showed that moist air favors growth, delays flowers and 

 fruit, and suppresses hairs. 



Experimental plants grown in moist air were characterized by 

 simplification of the palisade system and increase of the intercellular 

 spaces of the spongy parenchyma, reduction of vascular tissue, delayed 

 hgnification, enlargement of epidermal cells, great enlargement of 

 cortical and pith cells, formation of intercellular spaces where they are 

 normally lacking, delayed development of cork, and a reduction of the 

 number of stomata. 



In contrast with this, dry air retards growth (nanism) and favors in 

 high degree the development of hairy coverings. It induces a great 

 development of the root system and hastens formation of flowers and 

 fruit. The following anatomical changes are among the effects of dry 

 air: reduction in diameter of epidermis cells and in width of cortex and 

 pith; hastening of the formation of sclerenchyma, cork, and wood; 

 thickening of leaves, especially by increased number of rows of palisade 

 cells; and an increase in number of stomata. Dry air induces xero- 

 morphy. Transpiration is, however, such a complex process that the 

 relations between humidity and the income and outgo of water are 

 difficult to grasp. 



