390 Comparative Physiology. 



capable of such extension, really exists, it ought not to be con- 

 fined in its opex^ations by an already full and perfect formation 

 of parts, but should have only the rudiments of organs, in a 

 plastic condition, capable of transformation. The tendency of 

 the cambium, and all semiorganised matter, to throw off a 

 membrane when exposed, would seem to countenance such an 

 opinion, though it may only be an effort of vitality to cover the 

 exposed parts with a skin. In the Gardener' s Chronicle, 

 some time back, it was noticed, on the authority of accounts 

 from Egypt, that in some situations it had been found possible, 

 by slicing and uniting seeds of nearly allied species of the 

 genus Citrus, &c., to produce plants which, in their develope- 

 ment, partook of the nature of the different species united. 

 The most curious case of developement that has come under 

 my own observation is that of the Cytisus Adam/, or purple 

 laburnum. It has been said by some to have been produced 

 from the union of the two barks of the bud of a Cytisus pur- 

 purea, inserted in a stock of the common laburnum, and to be 

 the product of an adventitious bud developed where the two 

 barks unite ; by others it is said to be a true hybrid from seed. 

 The flowers are generally of a greyish purple colour, the leaf 

 and habit of growth resembling the common laburnum. Some 

 of the branches have been found to sport off to the common 

 laburnum with yellow flowers ; but the most unaccountable 

 circumstance is, that some plants which were for some time 

 grafted and pruned, and had the ordinary strong growth and 

 foliage of the laburnum, have, at the place where ordinary 

 branches had been pruned off, been found to develope shoots 

 of the true Cytisus pui'i^urea, which is so strikingly different in 

 habit and foliage from the laburnum. A union of indusiums 

 would best account for the accidental protrusion of parents*, 

 though it might puzzle us to account for the manner in which 

 they could be united in the purple laburnum, so as to affect the 

 colour of the blossom only. The alteration of the pictures of 

 the imagination, or idea of the living principle (of Miiller), 

 might be supposed more capable of change, and to embrace a 

 wider range of variations. To talk, however, of things we liave 

 no means of demonstrating is apt to bewilder, and lead us away 

 from the truth. The doctrine of preformation of parts is, to a 

 certain extent, vmdoubtedly correct, as any one may see for 

 himself by dissecting buds, bulbs, &c. ; beyond this, however, 

 we have no correct data to go by. The habit or manner of 

 growth of trees belongs also to developement, and when we see 

 round-headed trees change to fastigiate forms, from no cause we 

 can perceive, as in the Exeter elm, &c., it might puzzle us to 



* The two parents (Cytisus Laburnum and C. purpurea, which form C. 

 Adami) are found at times to protrude from branches of C. Adam?, as if not 

 completely united, but only held in mixture. 



