XXIV] HABIT 195 



tomy" arises in such a scheme, by temporary arrest of the apical growth. 

 The tip of the leaf {c, d), and even of the cotyledon itself {e, e) may retain 

 its circinate coil, while the pinnae below it expand. It is clear in Fig. 473, e , 

 that the two equal pinnae are not themselves the result of equal dichotomy, 

 but pinnae sympodially produced according to the scheme illustrated in {a). 

 The whole apical region is delayed in its development: but though dormant 

 it still possesses a power of indefinite further growth. This scheme may be 

 repeated in each pinna or pinnule, giving the key to the indefinite foliar 

 development so often realised in the genus when forming thickets on 

 savannahs. The degrees of branching of the leaves have been made the 

 basis of systematic division of the section Dicranopteris {Mertensia) into four 

 sections (Fig. 474). A further elaboration is described by Goebel in a mode 

 of protection of the resting bud, found in some species (Goebel, Organography, 

 Engl. Edn., p. 318). The pinnules that stand nearest to the apex develope 



Fig. 474. Scheme of branching of the leaf in the our sections ot Mertensia Willd. —{Dicranopteris). 

 They illustrate different results of the interruption of apical growth in the pinnate leaf. (After 

 Diels, from Engler and Prantl.) 



as scales covering it; they have sometimes been mistaken for adventitious 



or aphlebioid growths. As a matter of fact the whole structure of the leaf, 



including its protections, can be referred to a normal pinnate development 



altered by temporary arrest of the apex, and by precocious development of 



certain pinnae or pinnules. As regards the pinnules themselves two broadly 



distinct t}'pes are seen : that with relatively long flat pinnules {^Dicranopteris = 



Mertensia), and that having short and semicircular pinnules with convex 



upper surface {Eu-Gleiclietiia and Platysoma). The latter is probably a 



derivative xerophytic type. 



Dermal appendages of various forms are found in the Family. They vary 



from simple uniseriate hairs to flattened scales, and they are sometimes seated 



on emergences of considerable bulk. In Eii-GleicJienia broad scales protect 



the dormant leaf-apices and appear also on the leaf-bases and rhizomes: but 



simple hairs are associated with them: both fall off early in many species. 



The same holds for most species of \Dicranopteris\ but in G. linearis and 



pectinata, which we shall find to be distinguished strongly from the rest of 



the genus by marked characters of stele and sorus, scales are absent, and 



13—2 



