70 THE FACTORS [Part I 



statement shows how considerably growth is favoured by rarefaction of 

 the air : — • 



GROWTH UNDER DIFFERENT CONDITIONS OF ATMOSPHERIC 

 PRESSURE (after Jaccard). 



(R. represents growth in air at a pressure of 15 cm. ; O. at the normal atmospheric pressure.) 



R. O. 



1. Jerusalem artichoke. Tubers with shoots 1 cm. long, 



in 8 days ...... 



2. Vicia Faba, 3-4 cm. high, in 8 days . 



3. Oxalis crenata, tubers with two tall shoots . 



4. Bellis perennis, plant 3-4 cm. high, in 15 days 



5. Violet, plant 3 cm. high, in 15 days . 



6. Onions, with shoots 3-3 i cm. high, in 10 days 



When air less rarefied than the above is used, correspondingly less marked 

 results follow, but in spite of the greater importance of moderate atmo- 

 spheric rarefaction in relation to plant-life, this has secured scarcely any 

 attention from Wieler and Jaccard. A pressure of 35 cm. was employed 

 in one solitary experiment conducted by the latter observer on wheat- 

 seedlings. The plants experimented on attained in twenty-three days 

 a length of 20 cm., instead of 17! cm. at ordinary atmospheric pressure. 



Air that is as rarefied as that used in most of Wielcr's and Jaccard's 

 experiments occurs in nature only at the highest summits of the Himalayas, 

 for example at an altitude of 8,839 meters on Gaurisankar, where, if we 

 assume that the temperature of the air at the sea-level is 25 , a pressure of 

 26 cm. prevails. An atmospheric pressure of 35 cm., under which the 

 above-mentioned experiment with the wheat-seedlings was carried out, 

 corresponds to an altitude of about 6,coo meters, at which, in Thibet, 

 a stately flowering plant, Saussurea tridactyla, has its normal habitat : this 

 plant is discussed and illustrated in the section of this book dealing with 

 alpine vegetation. There appears therefore to be no reason why some 

 vegetation, even if only cryptogamous, should not occur at still greater 

 altitudes. In any case however, judging from the information available, 

 there are very few plants occurring at altitudes such that the rarefaction of 

 the air would cause their growth to be appreciably more rapid than in the 

 low land. For a definite reply to the question we must await the results of 

 experiments on typical alpine plants. 



The variations in the atmospheric pressure at different altitudes, even if 

 not directly, yet indirectly are of vast physiological importance, since 

 humidity, temperature, and light depend upon their magnitude. The 

 changes that these last-named factors undergo owing to a reduction in 

 atmospheric pressure are responsible for the influences exerted by the alpine 

 climate on vegetation, as will be described further on. 



