126 ON THE EXPOSURE OP PLANTS. 



being no general rule to guide. If, indeed, we are told that it is an 

 annual from a warm country, we may safely conclude that it will 

 succeed in this climate during summer, as many tropical annuals do. 

 Or, if it be a perennial herb from the same country, we may find it 

 answer with us if it he only protected from frost. But if tropical 

 shrubs or trees are brought to us, we cannot, from any external mark, 

 judge whether they are liable to be killed by frost or not. If they 

 shed their leaves in winter, it is only a sign that they are winter- 

 resting plants, not that they are hardy ; because there are several 

 tropical plants which are deciduous, as, for instance, the silk cotton- 

 tree (Bombax ceiba) ; and many evergreens are as hardy as those 

 that shed their leaves. 



We often fail in preserving tender plants from inattention to local 

 circumstances. We are liable to mistake shelter for warmth. Frost 

 and the north and east winds are most dreaded in this country. A 

 southern exposure, whether for the abode of animals, or a station for 

 vegetables, is always considered the most eligible, merely, perhaps, 

 because it is the most agreeable to our own perceptions. But in 

 respect of vegetables we often err in this matter, both in choosing 

 sheltered situations and southern exposures. 



Cold (or rather cold air) is always most intense in humid situations, 

 because there is the most copious evaporation. Such situations, in 

 this country, are either on the tops of clayey hills, or in the lowest 

 valleys, where there is either a lake, river, or brook. These low 

 grounds are nearer the main springs, and often abound with them, 

 whence exhalations are ever rising, though imperceptible ; of course 

 such a valley must always be more chilly, and more subject to keen 

 frost than any drier or more elevated situations. Such glens, pro- 

 vided they are open to the south, are chosen as the most suitable fur 

 tender exotics, merely because they are more sheltered from the 

 northern blast. In the summer, indeed, such a locality is most 

 favourable to the quick and strong growth of every plant. The air, 

 being generally calm and moist, conduces to vigorous expansion ; and 

 the very coolness of a summer's day or night, as felt in such places, 

 is most propitious to luxuriant vegetation. These circumstances, 

 however, instead of being beneficial to tender exotics, have a directly 

 contrary effect ; the summer excitement only renders them less able 

 to bear the frosts, which fall upon them with redoubled intensity in 



