340 Comparative Physiology. 



place. It might, however, as well be theoretically argued, that 

 the same heat that caused the expansion of the air would again 

 neutralise the danger by thawing the juices. To a certain 

 extent it is true, that covering and watering in the morning 

 will do good ; the vegetative organs, weakened by the damage 

 they have sustained, are unable to bear the full stimulus of 

 light and heat, and will not suffer so much when covered. 

 The water, if thrown on before the hoarfrost is thawed, 

 washes it off and restores the temperature of the shoot; it 

 also invigorates and enlivens the healing process of vitality in 

 any of the parts that are sound. After a severe frost in May, 

 the plants are always found to revive much sooner when the 

 frost takes the air, as it is called, and moist weather ensues ; 

 those revive soonest that have most spare buds. It is folly, 

 however, to tell the practical man that no danger ensues till 

 the heat of the next day begins to operate: I have myself 

 often watched the plants with anxious heart on such mornings, 

 and uniformly found, that, wherever the leaves had blotches 

 of a darker green, betokening the extravasation of juices from 

 ruptured cells, these leaves were sure to perish. The dif- 

 ference, however, is not perceptible to the ordinary observer, 

 till the heat of the day ensues ; and hence the opinion that the 

 danger commences then. Those that do not decidedly exhibit 

 these blotches before the rising of the heat seldom perish alto- 

 gether, though they sometimes appear whitened in the colour, 

 and scathed in their appearance ; and it is to such states of 

 damao;e that coverina; and water will be found most beneficial. 

 The continued effect of low degrees of heat, though perhaps not 

 below the freezing point, and not attended with so sudden 

 injury, is also, however, very baneful. When water is in excess 

 in soils, greater evaporation and cooling of the tissue in the 

 young shoots must ensue ; and the same will take place in long- 

 continued cold weather. Plants, from being more exposed in 

 their vital parts when growing than animals, are more apt to 

 suffer from cold. So great a degree of heat is not necessary ; 

 but that they are possessed of a certain degree of internal heat 

 might be inferred from the chemical and electrical processes 

 going on Avherever life is active. The chemical transformations 

 produced in the preparation of the latex should evolve heat. It 

 is a general belief, that most of the food absorbed is reduced to 

 carbonic acid, in order that from the carbonic acid may be 

 eliminated the nascent carbon, by the disengagement of oxygen 

 in the leaf; from which, united to the oxygen and hydrogen 

 from water, and the nitrogen from ammonia, are formed most 

 of the products found in the latex or proper juice, especially 

 that azotated matter called vegetable fibrine, from which are 

 formed most of the vegetable tissues. All these processes, united 

 to the vital action of the organs in assimilating the products of 



