428 



VASCULAR CRYPTOGAMS. 



and vertical stems, the intemodes are usually undeveloped, and the leaves so crowded 

 that no free portion of the stem remains uncovered, or only a very inconsiderable 

 one ^ The leaves of Ferns are usually characterised by a circinate vernation, and 

 they only unroll in the last stage of their growth ; the mid-rib and the lateral veins 

 are curved from behind forwards. The forms of the leaves are among the most 

 perfect in the whole vegetable kingdom ; they manifest an enormous variety in 

 their outline, the lamina being usually deeply lobed, branched, or pinnate. In com- 

 parison with the stem and the slender roots they are mostly very large, and some- 

 times attain extraordinary dimensions, even a length of from 6 to lo feet (as in 

 Pteris aquilina and Cihotium). They are always stalked, and continue their growth 

 at the apex for a long time ; the leaf-stalks and the lower parts of the lamina are 

 often completely unfolded while the apex is still growing (as in Nephrolepis). This 

 apical growth is not unfrequently interrupted periodically {vide infra) ; in Lygodium 



Fig. -i.^^.— Pteris aquilina, a part of the underground stem with leaves and bases of the leaf-stalks (reduced about one- 

 half) ; / older portion of the stem bearing the two bifurcations // and //', jj the apex of the weaker branch //; beside it the 

 youngest leaf-rudiment 8 ; 1-7 the leaves of this branch, one being developed in each year ; 1-5 the leaves of earlier years, which 

 have already died off at some distance from the stem ; 6 the leaf of the present year with unfolded lamina, the stalk having been 

 cut off; 7 the young leaf for next year ; at the apex of the stalk is the lamina still very small and entirely clothed with hairs. The 

 leaf-stalk / bears a bud /// a, which has developed a leaf* that has already died off. The more slender filaments are roots. All 

 the parts shown in the figure are subterranean. 



the leaf-stalk or the rachis resembles a twining stem with long-continued growth, the 

 pinnae presenting the appearance of leaves. The amount of metamorphosis of the 

 leaves is, notwithstanding, very inconsiderable ; on the same plant the same forms 

 of leaves, mostly foliage-leaves, are constantly repeated ; scale-like leaves occur on 

 underground stolons {e.g. in Siruthiopteris germanicd), and in many cases the fertile 

 leaves (those which bear sporangia) assume special forms. Such differences as 

 occur in most Phanerogams are not found in the development of the leaves of one 

 plant ; Plaiycerium alcicorne must, however, be mentioned, as having the foliage- 

 leaves alternately developed as broad plates closely applied to the supporting surface 

 and as long dichotomously branched ribbon-shaped erect leaves. 



Among the various forms of trichomes of Ferns those termed Ra?7ienia are 



* From the change of form and of size presented by the older cicatrices Brongniart concluded 

 that the stems of the Tree-ferns continue to grow in length (and in thickness?) for some time after 

 the leaves have fallen off. 



