INFLUENCE OF HEAT ON PLANTS. 77 



some. Thus the Oerasus Virginiana grows in the Southern States of 

 North America as a noble tree, attaining one hundred feet in height ; 

 in the sandy plains of the Saskatchawan, it does not exceed twenty feet ; 

 and at its northern limit, the Great Slave Lake, in lat. 62, it is re- 

 duced to a shrub of five feet. Another curious effect of heat is shown 

 in its influence on the sexes of certain Monoecious flowers ; thus Mr. 

 Knight mentions that Cucumber and Melon plants will produce none 

 but male or staminiferous flowers, if their vegetation be accelerated by 

 heat ; and all female or pistilliferous, if its progress be retarded by cold. 



108. The injurious influence of excessive Heat can be, to a certain 

 extent, resisted by Plants, -through the cooling process kept up by the 

 continual evaporation of moisture from their surface. But the power 

 of maintaining this cooling process entirely depends upon the supply of 

 fluid, with which the plant is furnished. If the supply be adequate to 



i the demand, the effect of heat will be to stimulate all the vital opera- 

 tions of the plant, and to cause them to be performed with increased 

 energy ; though, as we have already seen, this energy may be such as 

 i to occasion a premature exhaustion in its powers, by the excessive luxu- 

 i riance which it occasions. But if the supply of water be deficient, the 

 I plant is burnt up by the continuance of heat in a dry atmosphere ; and 

 I it either withers and dies, or its tissues become dense and contracted, 

 j without losing their vitality. Thus it has been remarked, that shrubs 

 growing among the sandy deserts of the East, have as stunted an ap- 

 pearance as those attempting to vegetate in the Arctic regions ; their 

 leaves being converted into prickles, and their leaf-buds prolonged into 

 thorns instead of branches. The influence of excessive heat in destroy- 

 ing life, can sometimes be traced through the direct physical changes 

 which it occasions in the vegetable tissues. Thus it has been ascer- 

 tained that grains of corn will vegetate, after exposure to water or 

 vapour possessing a considerable degree of heat ; provided that heat do 

 'not amount to 144 in the case of water, and 167 in that of vapour. 

 .At these temperatures, the structure of the seed undergoes a disorga- 

 nizing change, by the rupture of the vesicles of starch which form a 

 large part of it; and the loss of its power of germinating is therefore 

 i readily accounted for. The highest temperature which the soil usually 

 possesses in tropical climates, is about 126, though Humboldt has once 

 observed the thermometer rise to 140. Seeds imbedded in such a soil, 

 (therefore, may not lose their vitality, although they will not germinate 

 ! in such temperatures. The temperature most favourable to germination 

 probably varies in different species, and is one of the conditions that 

 i produces their adaptation to different climates. Thus it appears that 

 | Corn will not germinate in water at a higher temperature than 95, 

 ! whilst Maize will germinate in water at 113; and, as is well known, 

 Maize will flourish in countries in which Corn cannot be grown. 



109. We must not confound the power which Plants possess of vege- 

 \tating ', or exhibiting vital activity, under widely-different degrees of 

 temperature, with the power of retaining their vitality in a dormant 

 condition, which many of them possess in a very remarkable degree. 

 When the external temperature is much below the freezing-point, it is 

 'impossible that any vegetating processes can go on ; since the Plant 



