LETTER Xy. 



165 



we are prepared to say that every separate portion of this mass, 

 f^hich can maintain an independent existence, is to be regarded 

 is endowed with a distinct individuality, i^ow, the duration of 

 ;his cellular stem of the Cactace^ is extremely prolonged, its 

 ife being very slow ; so that there are undoubted instances of 

 slants of this order continuing to exist for 100 years ; and 

 :heir probable term of life is very much longer." * 



12. I can see nothing in what is here said regard- 

 ng the CactacecB at all at variance with my theory. 

 But I see much which, on the principle of Exceptio 

 urohat regidam," goes to give support to it. The 

 constitution indeed of the Cactus is less complex, — 

 that is to say, it contains fewer distinct parts than 

 Drdinary plants, and, in particular, it is destitute of 

 iistinct and ordinarily constituted leaves. But, ob- 

 serve, whatever be the special object of its existence, 

 it is plain that it was not made for the production of 

 timber. And accordingly, whatever other reasons 

 there may be for the simplicity of its structure, — and 

 while it has in the foliaceous surface of its succulent 

 stem just that sort of leaf which performs the only 

 office that this plant needs, viz. the elaboration of 

 sap, — it has not that sort of leaf which is required for 

 that other office which (as we have already seen and 

 shall see again by and bye) the leaf serves to timber- 

 producing plants, to wit, a leaf in itself distinct and 

 special, possessed of ligneous tissue, and, in some way, 



* lUd, p. 904. 



