114 THE PHENOMENON OP 



the leaf of the Phanerogamia ; but that Schleiden's theory 

 of leaf-formation is only partly warranted, even in this 

 second stage of growth, is shown by Grisebach's dis- 

 covery, that several points of vegetation or foci of develop- 

 ment frequently present themselves within the leaf, and 

 an intercalary cell-formation occurs not only at the base, 

 but at various points.* Even the last stage of growth of 

 the leaf, depending on the mere expansion of the cells, 

 does not always, or in all parts of the leaf, progress in 

 the descending (centripetal) direction, as followed indis- 

 putably, even before Grisebach's researches, from the 

 observations of Steinheil (especially on the leaf of the 

 Magnolias] and Munter (on pinnate leavesf), although the 

 difference between the growth by cell- development and 

 cell-formation was not actually distinguished in them, j 



The general and partial gradual unrolling, advancing 

 toward the apices, and the final expansion of the Fern- 

 leaf connected with this, is a well-known phenomenon, 

 which has been incorrectly regarded as an evidence 

 against the foliar nature of this organ. 



* Grisebach, ' Beobachtungen iiber das Wachsthum der Vegetations 

 organe in Bezug auf Systematik.' (Weigmann's Archiv, 1844, 134.) 



f ' Observationes sur la mode d'accroissement des feuilles.' (Ann. des Sc. 

 nat. 1837.) 



| ' Beitrag zur Lehre vom Wachsthum der Pflanzen,' Bot. Zeit., 1843, 

 p. 785. 



Especially remarkable, in this respect, are certain Ferns, in which the 

 points of the leaves are never totally unrolled, as in the narrow, simply 

 pinnate Platyzoma microphyllum, and the Jamesonite resembling them in 

 Labit (e.g. J. imbricata, Hook, et Grev., t. 178, scalaris, cinnamomea, verti- 

 calis Kunze, 'Die Farrnkrauter in Colorirteri Abbildungen,' t. 71 and 82). 

 Still more remarkable are many species of Gleichenia and Mertensia, in which 

 the development of the leaf is arrested above the first pair of pinnules (and 

 in multipinnate rudiments often repeated in several degrees of the ramifi- 

 cation), so that the point, seeming to form a bud in the bifurcation, either 

 remains permanently undeveloped, or is only unfolded in the succeeding 

 season, and then again in like manner only imperfectly. This sectional 

 development of the leaf, in which we behold one of the most remarkable 

 phenomena of Rejuvenescence within the leaf itself, appears capable of 

 lasting through many years, on which head it would be very desirable to 

 obtain more accurate information from observers in the native countries of 

 these Ferns (see Kaulfuss, ' Das Wesen du Farrnkrauter'), 1827, p. 36). 

 That the leaves of these Ferns do not possess, however, as might seem, an 

 unlimited growth, is proved by the leaves of the young Mertenaiee with a 



