194 APPLICATION OP PBINCIPLES. 



Clianthus puniceus (6), Gloxinia speciosa (c). In these, 

 and all such cases, the first thing that happens is an 

 excessive development of the cellular tissue, which 

 forms a large convex "callus" at the base; from 

 which, after a time, roots proceed; and by which 

 eventually a leaf-bud, the commencement of a new 

 stem, is generated. 



It is not surprising that leaves should possess this 

 quality, when we remember that every leaf does the 

 same thing naturally while attached to the plant that 

 bears it ; that is to say, forms at its base a bud which 

 is constantly axillary to itself. Leaves, however, 

 have not been often employed as the means of pro- 

 pagating a species; and it is probable that most 

 leaves, when separated from their parent, are incapa- 

 ble of doing so, for reasons which we are not as yet 

 able to explain. The most common case of their em- 

 ployment is in the form of the scales of a bulb, which 

 will, with some certainty, produce new plants under 

 favourable circumstances. Those circumstances are, 

 a strong bottom heat, moderate moisture, and a rich 

 stimulating soil. 



When plants are produced by leaves under ordinary 

 circumstances, the conditions most favourable to their 

 doing so are of the same nature. A moderate amount 

 of moisture prevents their dying from perspiration or 

 perishing from decay ; a good bottom heat stimulates 

 their vital forces, and causes them to exercise what- 

 ever power they possess ; and, in addition, they are 

 covered by a slightly shaded bell glass, which main- 

 tains around them an atmosphere of uniform humi- 



