76 PERIODIC PHENOMENA OF VEGETABLE LIFE. 



rnoting the evaporation of leaves, has been frequently demonstrated 

 by direct experiments on the growth of plants. Miinter,* I farting,! 



and others, have proved that the growth of plants is greater by 

 day than by night; that under a decreasing intensity of develop- 

 ment, it may be confined to the day-time, and that it is more active 

 in direct sunshine than in the shade. 



Nevertheless, the heat which a plant receives is different from 

 the mean temperature obtained by the observation of a thermometer 

 suspended in the shade ; for plants have usually their free parts 

 exposed to the action of the sun,]; whilst their roots partake of the 

 temperature of the soil. In shady situations, the mean temperature 

 of the air is lower, and the extremes are nearer together, than in 

 sunny places ; for the maximum is much lower in the shade, and 

 the minimum is also not quite so low, owing to the diminution of 

 nocturnal radiation. This contrast between the shaded and sunny 

 situations is greater during the warmer than during the colder 

 portions of the year, and increases greatly in more elevated situ- 

 ations in comparison with lower ones. Yet, variations even in the 

 mean temperature of the air, as indicated by the thermometer in 

 the shade, exercise great influence on the development and general 

 growth of plants. 



In the scale of heights in the Alps, the variations, as we have 

 already observed, diminish with greater elevation, but more 

 because at greater altitudes the heat is less, than because the cold 

 is greater : the decrease in temperature is greater in summer than 

 in winter. 



The great influence which the climaterie character and mode of 

 repartition of temperature exercise on the development of plants 

 may be readily perceived, if we endeavour to draw synchronistic 

 lines ; that is, if we connect those places where certain phenomena 

 of vegetation occur at the same time. These lines, in extensive 

 continents, do not, as observed by Quetelet, coincide with mean 

 annual temperatures, and are neither parallel, nor do they show 



* Observations phytophysiologicos. Berlin, 1841. See Schlechtendahl's 

 Linncea,vo\.xv. pp. 200—242 ; Mohl and Schlechtendahl's BotaniscJie Zeitwag, 

 vol. i. pp. 69, 753, &c. 



f In F. Van Hccven and De Vriese's Journal, Leyden, 1842, vol. iv. pp. 296 

 — 348, extracted in Mohl and Schlechtendahl's Journal, vol. i. pp. 90 — 102. 

 For the relations of the development of plants to external conditions, as 

 well as for the physiological relations of these phenomena, ses De Candolle's 

 Physiologic Vegetale, vol. i. Schleiden's Principles of Scientific Botany, 

 second edition, 1845, vol. ii. pp. 494 — 503. 



X For the great influence on vegetation of the direct exposure to sunshine, 

 see Von Humboldt's De Distributions Qeographica Plantarvm, 1817, p. 163. 



