70 ON YARTKTIES, HACKS, SUB-SPECIES, AND SPECIES. 



the external world in which they have to live, we proceed to 

 discuss the questions of degeneration and persistence of the 

 varieties of our cultivated plants : but we must first give a short 

 account of the general methods of propagating vegetable species 

 considered, in the individuals representing them, as simple 

 varieties, races, or sub-species. 



Propagation of Vegetable Species. 

 Two methods are usually employed for the propagation of 

 vegetable species : 



A. By the simple division of an individual representing a 

 species, sub-species, race, or a simple variety. 



B. By sowing the seeds of an individual. 



A. Propagation by Simple Division. 



This goes on by the development of an organized part sepa- 

 rated from a living individual either naturally or artificially. 



1. Propagation by Layers. 



■ A stem or branch in a horizontal position, and partly covered 

 with damp earth or moss, produces roots, but remains attached to 

 the parent plant ; if after a time the stem or branch which has 

 rooted be separated from the stock, we have a layer representing 

 that stock. 



2. Propagation by Cuttings, 



A stem, branch, or leaf, separated from its parent and placed 

 •in earth, produces roots and becomes a living individual obtained 

 by ctitting. 



If the cutting is composed of a young shoot attached to some 

 old wood, it; bears the name of Crossette; this is very common 

 in vineyards. 



3. Propagation by Grafting. 



Everybody knows that an essential or adventitious bud taken 

 from a living plant and placed upon another in such a way that 

 the youngest and most analogous tissues in each are in contact, 

 the l3ud and the plant on which it is placed, grow together ac- 

 cording to tlie law of homceozygy {Journal des Savants, 1840), 

 and tiie bud, then called a scion, is developed witlj the characters 

 of the plant from which it was taken, by means of the nourish- 

 ment supplied by the stock on which it is placed. 



4. Propagation by Tubercular Buds, 



The stems of most plants produce tubercular buds which can 

 be developed either at their subterranean or aerial part. As an 



