THE METABOLISM OF PLANTS. 313 



which is going on, and at the same time to raise perceptibly 

 the temperature of the plant or of the organ. We shall see, 

 when we come to consider the loss of heat by the plant, 

 that a considerable amount of heat must be continually 

 evolved in it, or else its temperature would be found to 

 be lower as compared with that of the surrounding medium 

 than is actually the case. 



We may now pass on to the loss of heat by plants. In 

 the first place subaerial organs lose heat by radiation. A 

 conclusive proof of this is afforded by the familiar fact that 

 dew and hoar-frost form so readily on plants. The signifi- 

 cance of this is that, in consequence of loss of heat by 

 radiation, the temperature of the plant sinks below that of 

 the atmosphere. It is on this account that plants often 

 suffer from frost when the temperature of the air is actually 

 above the freezing-point. Plants or parts of plants growing 

 in water or in the soil lose heat by conduction. 



This subject was first investigated by Wells, and more recently by 

 Boussingault and by Maquenne. Boussingault mentions that a thermo- 

 meter placed on grass on a clear night often indicates a temperature 

 7-8C. below that of the air. Maquenne states that leaves radiate 

 almost as much as lamp-black. 



In the second place, subaerial organs lose heat in connex- 

 ion with transpiration. Transpiration, we have seen (Lec- 

 ture VII), consists in the exhalation of watery vapour by the 

 plant. This involves the conversion of water into vapour in 

 the plant, and for that a certain amount of heat is necessary ; 

 and it involves, further, a transpiration-current through the 

 root, stem, and branches, which has a subsidiary cooling effect. 

 Thus Rameaux found that, when two similar branches, 

 one of which was deprived of its leaves, were exposed to the 

 sun and supplied with water, the temperature of the interior 

 of the branch which bore leaves was 10 C. lower than that 

 of the branch which had been deprived of its leaves. 



In conclusion we will briefly consider the temperature of 

 plants. A number of factors cooperate to determine it : first, 

 the activity' of the evolution of heat in the plant; secondly, 

 the temperature of the surrounding medium; thirdly, the loss 



