CLIMATIC FACTORS 89 



May starch gains the ascendancy. This change in form of the 

 carbohydrates greatly influences the hardiness of the leaves. 



As long ago as 1727 Hales, discussing the injury to plants by cold, 

 said, "Oil is an excellent preservative against the injuries of cold"; 

 and more recently Harvey (1930) has discussed the chemical reasons 

 for such protection. Not only do such northern trees as species of 

 Pinus, Betula, and Prunus form oil from carbohydrates during the 

 autumn and winter, but in such genera as Lmum, Helianthus, and 

 Cannabis the same species show higher oil content in lower as com- 

 pared with higher temperatures. The action of the oil seems to be 

 concerned with the maintaining of the fluid condition of the proto- 

 plast at low temperatures. 



Temperature Changes. — Changes of temperature are the more 

 dangerous to plants the more suddenly they occur. Adjustment to 

 gradual changes of temperature is possible within limits (c/. Harvey, 

 1918). In nature there is generally a sharp distinction between hardy 

 communities which withstand large changes of temperature and those 

 which are restricted to a narrow range. To the first class belong the 

 Loiseleurietum cetrariosum of ridges exposed to wind, the Caricetum 

 firmae, and the Elynetum. These withstand —30° to — 40°C. in 

 winter without injury, while on clear summer days the surface soil is 

 heated to 50°C. or more. The thickets of Rhododendron and Vaccin- 

 ium of the Rhodoreto-Vaccinion are communities confined to narrower 

 amplitudes of temperature. 



Schade (1917) reported the extremes of temperature to which a 

 patch of Pohlia nutans was subjected during five years, in the Teufels- 

 schliichten of Elb sandstone mountains, on a northeast slope. This 

 plant, fully exposed to the sun, endured without harm an annual range 

 of temperature of 66.5°C. (maximum 56.8°, minimum — 9.8°C.). The 

 temperature of a liverwort sod {Leptoscyphus taylori, Calypogeia 

 neesiana, C. trichomanoides, C. media, Odontoschisma denudatum, etc.) 

 on a shaded rock face was much more equable. The annual range was 

 only 23° (maximum 17°, minimum —6°). Two local climates of quite 

 opposite nature here stand side by side. 



It is naturally much easier and more satisfying to work out such 

 contrasts than to determine with painstaking care the optimum 

 temperature relations of a definite plant community. 



Influence of Vegetation upon Temperature of Air. — Vegetation 

 tends to moderate the temperature. This equalizing effect increases 

 with number and height of the layers. Under communities of two or 

 more layers the extremes of temperature are nearer together: the 

 maxima are lower, the minima higher than in the open. Hence the 



