EDITOR'S PREFACE. xiii 



rigour of a northern winter, which destroys the life of her- 

 baceous plants, either altogether, as is the case with annuals, 

 or at least to the surface of the ground, as happens to our 

 humble perennials. 



It is obvious that the former will draw their juices from a 

 zone which is quite inaccessible to the ordinary vicissitudes 

 of climate ; and when, in addition to this provision, we ob- 

 serve the trunk, as in the instance of the Birch, encircled 

 with numerous layers of bark, or, as in the Pine tribe, re- 

 plete with resinous juices, which are not liable to congeal, 

 all the conditions favourable to the resistance of cold will 

 be seen united in these inhabitants of the North. 



Herbaceous plants, on the contrary, being sensible of 

 every change of temperature, can exist only where neither 

 the heat nor the cold is very excessive; and therefore 

 abound most in the Temperate Zones, diminishing in num- 

 ber and variety both as we advance to the south or to the 

 north of this middle region. 



Nor would it appear surprising, if the stimulus of heat 

 imparted to the productions of a tropical region that greater 

 size and bulk, which distinguish them so frequently from 

 plants of the same family met with in colder regions. 



it would seem but natural that the Tree-ferns of the 



