OP PROPAGATION BY LEAVES. 



193 



sisted of medulla, alburnum, bark, and epidermis ; 

 and as the leaf ; itself, during the growth of these, in- 

 creased greatly in weight, the evidence that it gene- 

 rated the true sap which was expended in their for- 

 mation appears perfectly conclusive." 



In our gardens, we know of many other cases of 

 the same kind. Hoya is a common instance, and three 

 others are here figured (Jig. 18) ; viz., Gresnera (a), 



18 



