REJUVENESCENCE IN NATURE. 113 



The tendrils of the Cucurbitacese, certainly belonging 

 to the leaf-formation, behave, leaving out of view the 

 suppression of the blade-formation, exactly in like man- 

 ner, and the pinnate leaves of the Dicotyledons have not 

 only a centrifugal expansion-growth of the petiolar 

 apparatus ; but the lateral leaflets unfold distinctly in 

 ascending order, while at the same time each individual 

 leaflet completes its own growth by a centripetal ex. 

 pansion.* 



In spite of the imperfection of our insight into the 

 laws regulating the formation of the leaf, we may at 

 least regard it as certain, that the leaves are developed 

 forth from an axis formed before them, and that this 

 axis, whenever leaf-formation occurs, is developed into a 

 stem after the rudiments of the leaves have come into 

 existence ; it may be further regarded as certain, that 

 the formation of the leaves is not simultaneous but suc- 

 cessive;! so that between two following leaves there 

 always exists at the outset, a separating structure, the 

 axis, permanent as such, whether it may be developed as 

 an internode or not. If we conceive the leaf in relation 



similar apiculated terminal structure, of which Martius ('Icones plant, 

 crypt.,' t. 60, ii) has represented an interesting series, but erroneously 

 referred to a distinct species (M. pumilci). The Uymenophyllum interruptum 

 figured by Kunze (' Anal, pteridograph., t. 30), likewise deserves mention 

 here. 



* The phenomenon may be seen especially well in the Mahoniae witli 

 pinnate leaves, the lower pinnules of which are already expanded, green, 

 and coriaceously hardened, while the upper are still scarcely half as large, 

 half folded together, reddish, and soft. 



(Vide also ' Proceedings of the Linnean Society of London,' May 6th, 

 1852, in a paper by Dr. Alexander on the leaf of Guarea grandifolia, Dene. 

 In this the upper leaflets are developed after the lower have fallen off, 

 periodical growth at the apex of the petiole going on for several years. A. H.) 



t This holds good even of the leaves of whorls, as transitions and sub- 

 stitutions of verticillate and spiral arrangements sufficiently testify. 

 Although recent observations on the development of the parts of the 

 flower describe the circles of papillae, forming the earliest representatives of 

 the circles of the flower, as coming to light simultaneously in all the segments, 

 we must remember that these papillae are by no means the earliest rudi- 

 ments of the leaf-formation, but that to solve this question by means of the 

 history of development, it would be necessary to go back to the origin of 

 the cells, or groups of cells, which subsequently rise up as papillae. 



