FILICES 



77 



The leaves of ferns stand either in two rows on the stem, or less 

 often in a single dorsal row, or the phyllotaxis is more complicated and 

 spiral ; they are never opposite or whorled. Each leaf originates from 

 a superficial cell of the growing point distinguished by a stronger swell- 

 ing of its outer cell-wall. The petiole is the first part of the leaf 

 formed ; the lamina then begins to appear at its apex, and itself 

 develops from the base to the apex. In many ferns the leaves of the 

 mature plant are characterised by the extraordinary slowness of their 

 development. In old plants of Pteris aquihna the formation of the leaf 

 commences fullv two vears before it bea;ins to unfold : at the com- 

 mencement of the second year only the leaf-stalk is as yet in existence ; 

 during the summer of the second year the lamina begins to develop at 

 the apex of the rod-like petiole, and may be found hidden beneath the 

 long hairs in the form of a minute disc. It then begins to bend down- 

 wards at its apex, and continues for a time in a pendent condition. It 

 is only in the spring of the third year that the elongation of the leaf- 

 stalk forces the lamina above the surface of the soil, and that the latter 

 begins to unfold. In Aspidium filix-mas the development is almost as 

 slow. The basifugal development of the lamina itseK is also extremely 

 slow in many ferns, the lower portions having long been fully formed 

 while the apex is still unfolding. In several genera, as Gleichenia (Sm.) 

 and Mertensia (Willd.), a periodical interruption occurs of the apical 

 growth, this intermittent development even extending over several years. 

 In Lygodium the primary pinnas remain unfolded after the formation of 

 a pair of pinnae of the second order, while the rachis of the leaf grows 

 without limit and resembles a twming stem, climbing in some cases to 

 the height of fifty or one hundred feet, the pinn^ themselves presenting 

 the appearance of leaves. In the anomalous Ceratopteris thalictroides, 

 which grows in water, the ultimate segments of the leaves are swollen 

 up in a pod-like manner. Goebel has shown (Ann. Jard. Bot. Buiten- 

 zorg, vii., 1887) that in the heterophyllous ferns, such as Platycerium 

 alcicorne and several species of Polypodium (L.), one form of leaf is 

 specially adapted, directly or indirectly, for the supply of nutriment to 

 the plant, and is sometimes furnished with special aquiferous tissue. 

 The stipular structures of the Marattiacese are quite peculiar among 

 Vascular Cryptogams. The leaves of ferns not unfrequently display a 

 tendency to branch dichotomously at the apex, but in other cases the 

 branching appears to be monopodial. The leaf-stalk has frequently the 

 power of producing adventitious leaf-buds (see fig. 49). In Struthiopteris 

 germanica these develop into long underground stolons covered with 

 scale-leaves, which at length rise above the surface, and besr at their 

 apex a rosette of ordinary leaves. Nephrolepis (Sch.) is also furnished 



