134 MISSOURI BOTANICAL GARDEN. 



From this it will be seen that at the surface the highest 

 temperature was 7.1° C. higher in the o[)m than in the w^ood, 

 while half a foot below the surface a difference of G.l° C. was 

 found. It will also be noted that in winter the tempera- 

 ture of the soil half a foot below the surface was considerably 

 higher in the wood than in the open. 



Not only are plants growing in woods protected from ex- 

 treme temperatures, but they also enjoy a fairly uniform 

 degree of humidity of the soil. Another factor which fre- 

 quently plays a great part in the presence of plants in certain 

 localities is the air current. This is true especially where 

 occur ravines such as we find at Cliff Cave. Just how 



m 



these air currents affect plant growth will be shown later. - 



im 



its intimate relation to plant distribution, is the ratio exist- 



by 



so7c 



the evaporation. t Naturally the nature of the plant growth, 

 etc., modifies this ratio to a great extent, as shown experi- 

 mentally by Livingston,! by means of his simple and in- 

 gonious atnioTiietor. 



A seeming gradual decrease 



§ 



ted a great and lasting influence upon the 



mc 



could be followed are unfortunately not available. 



Important climatological influences upon the flora are 

 exerted by the daily extremes of tc^nperaturC; especially 

 those occurring during the earlier part of the ycar^ which 

 act not only upon the plants themselves but also upon the 

 limestone rock. Alternate periods of freezing and thaw- 

 ing can be supported by the winter-rosettes of the hardiest 

 plants only. Differences of 30^ F. are not infrequent, and 



* Schimper, A. F. W. rflauzengcographie. 174. Jena. 1898. 



t Transeau, E. N. Forest Centers of Eastern America. Am. Nat. 39 : 



875. 1905. 



t Livingston, B. E. Evaporation and Plant Development. The Plant 



World. 10:260. Dec. 1907. — Evaporation and Plant Habitats. The 

 Plant World. 11:1. Jan. 190S. 



§ Trelease, W\ Thirteenth Annual Report of the Director. Rept. 

 Mo.Bot.Gard. 13:1G. 1902. 



