THE LEAF. 



225 



upwards ; it occupied a perfectly terminal position, 

 the uppermost youngest portion of the branch having 

 been pushed over into a lateral position (PI. XXIV) ; 

 in another branch the same peculiar double-leaf 

 structure occurred, but here the young, developing 

 portion of the branch had become completely extinct 

 and the double-leaf formed the veritable termination 

 of the branch (PI. XXV). Tins congenital union, 

 face to face, of two leaves in the manner described, 

 gives rise to a structure which is almost certainly 

 identical with those above described in Phlox, Saxi- 

 fraga, and Polygonum, but in a much more developed 

 condition. If we imagine the two ventral laminae 

 iu the Phlox-leaf becoming united at their apices and 

 greatly enlarged so as to equal in size, or nearly so, 

 the blade on which they are borne, the result would 

 be a structure quite similar to that in Buddleia. 

 Buchenau cites a case in the tobacco-plant of the 

 same phenomenon, but in this instance the leaves were 

 adherent by their lower (dorsal) surfaces, the apical 

 portions being free. The same thing has occurred in 

 • the bird's-lip flower (Strelitzia reginae) ; the two leaves 

 being coherent back to back for the whole length of 

 their blades and petioles.* 



This is one type of double terminal leaf in which 

 the constituents are coherent by their (upper or lower) 

 surfaces. In the type about to be mentioned the 

 union is marginal, being similar to the mode of forma- 

 tion of the cup- or salver-shaped double cotyledons 

 which occurs in some plants. De Vries figures seed- 

 lings of the snapdragon (Aittirrhinum majus) in one of 

 which there was such a double plumular leaf succeed- 

 ing the two normal cotyledons (PI. XXII, fig. 6 c); in 

 another seedling with three cotyledons the terminal 

 structure consists of a single blade with a basal pocket 

 (PI. XXII, fig. 6 a) ; it is, in fact, just the same struc- 

 ture as the "single" terminal leaf above mentioned, iu 



* The writer is indebted for these leaves to the generosity of Mr. W. H. 

 B. Fletcher, of Bognor. 



VOL. I. 15 



