No. 511] PRESENT PROBLEMS 7A r PLANT ECOLOGY 423 



no small decree modified and controlled by temperature, 

 l)iit who can put his finger on definite facts and say 4 'this 

 temperate forest is on this mountain in the tropics because 

 of temperature," or "this Alpine plant is a dwarf be- 

 cause of cold. ' ' Strongly as we may suspect such points, 

 we must be cautious about including them within the 

 realm of our positive knowledge. 



The peculiarities of Alpine plants have been thought 

 to be due, in part, to great daily variation in temperature; 

 that owing to the greater clearness and rarity of the 

 atmosphere, the plant is more exposed to heating by 

 intense sunshine by day, and cooling by more rapid radia- 

 tion at night. Since growth takes place particularly at 

 night, it seems evident that marked night cooling would 

 lead to reduced size. That this is actually true has been 

 proven by placing growing plants each night in the ice 

 chest. At present more light is chiefly needed as to what 

 temperatures are really experienced by Alpine plants over 

 night. Air temperatures, at any rate, are more equable 

 at high altitudes. In connection with the work in the 

 Selkirks during the past summer, two complete thermo- 

 graphic records were taken, one meter from the surface, 

 same hillside and exposure, at altitudes of 800 m. and 

 1,700 m., respectively. The daily maxima recorded at 

 the upper station were notably less than at the lower; 

 the nightly minima only slightly so. (Freezing point 

 was not recorded for many weeks.) Such data accord 

 well with the general result- of meteorologists. They 

 refer of course only to air temperatures. 



Briefly summarizing, we may say that our knowledge 

 of the relation of heat to mountain vegetation is not great. 

 The importance of extreme low temperatures has been 



