ON THE NATURALISATION OF PLANTS. 113 



.A layer of air is also retained captive between the ep- 

 idermis and the bark, which is, perhaps, a still better pre- 

 servative to the plant than the bark itself: it is a delicate 

 under-garment, which the stem wears beneath its more 

 cumbrous clothing of epidermis. 



Caroline. The epidermis was itself, I thought, a del- 

 icate covering to the internal layers of bark. 



Mrs. B. That depends upon the nature of the plant, 

 and the part to which it belongs. The epidermis of the 

 leaves and buds is delicate, but that of the stem and 

 branches of a venerable oak is of a very different descrip- 

 tion : the bark in general which covers the trunk consists 

 almost wholly of carbon ; which, being a very bad con- 

 ductor of heat, answers the double purpose of confining 

 the internal heat in winter, and excluding the external 

 heat in summer. 



The epidermis itself is sometimes single, sometimes 

 double or triple, but more commonly consists of a num- 

 ber of layers. So many as one hundred and fifty have 

 been counted in the epidermis of a tropical plant ; and 

 so great a number still remained that the calculation was 

 abandoned from the difficulty of completing it. 



At our next interview we shall examine, how far plants 

 will admit of being naturalised to a climate differing in 

 temperature from that in which they are indigenous ; and 

 what are the precautions necessary to be taken in trans- 

 planting them to a foreign country. 



CONVERSATION VIII. 



ON THE NATURALISATION OF PLANTS. 



Mrs. B. In estimating the effect of diversity of cli- 

 mate on plants, the point most important to be consid- 

 ered is the difference of temperature. The nature of the 

 soil, the air, water, and light, are circumstances compar- 

 atively trifling, compared with the abundance or defi- 

 ciency of heat. 



635. What is said of the air held between the bark and the epidermis 1 ? 

 636. What is said of the nature of the epidermis 1 ? 637. What is 

 said of the nature of bark, considered as capable of resisting the cold and 

 heaf? 638. Of how many layers does the epidermis consist! 639. 

 In estimating the effect of diversity of climate on plants what poiat is 

 of the moat importance to be considered 1 ? 

 10* 



