204 DERMAL AND OTHER NON-VASCULAR TISSUES [CH. 



Fig. 194. Pneumatophoresof/yfl^/i?_^r?a. 

 i, shows the persistent base of the leaf 

 of P. glauca, the abaxial face bearing 

 two rows of them (/). ii, the circinate 

 apex of the X&zH oi P. pycnophylla, show- 

 ing the pneumatophores {p) alternating 

 with the pinnae, and projecting through 

 the covering of mucilaginous hairs. 



In certain Ferns where the young parts are completely covered by dense 

 mucilage secreted from their hairy investment, the ventilation of the young 

 parts would be seriously prevented, were 

 it not that it is secured by outgrowths of 

 the nature oi pneumatophores. These are 

 well seen in Plagiogyria pycnophylla pro- 

 jecting beyond the mucilaginous sheath 

 which covers the young leaves. They re- 

 main to the period of maturity of the leaf, 

 and may still be seen functional in the 

 upper region, one at the base of each pinna 

 (Fig. 194). These are the "aerophorae" of 

 Mettenius(/v?r;z^(S:////;/^^/«,ii, 1858). Similar 

 structures are found elsewhere, particularly 

 in the very mucilaginous Ferns of tropical 

 rain-forests, such as Dryopteris {Polypo- 

 diiini) Thoinsonii, Jenm., and deciissata (L.), 

 Urban : also in Nephrodhivi stipellatwn, Hk. (see Haberlandt, Pflanzen- 

 anatoniie^ Aufl. iv, p. 400). Their occasional occurrence points to their being 

 special adaptations, which cannot be held of high value for phyletic discussion. 



The same is the case with those areas of surface specialised for secretion 

 of water, which are in direct communication with the vascular system, and 

 consist of a group of thin-walled epidermal cells, sunk in a shallow depression, 

 immediately above a bundle-ending. These are either uniformly scattered 

 over the surface, or placed in series along the leaf-margin. Examples are 

 seen in the leaves of species of Polypodmin^ Dryopteris, and Nephrolepis 

 (Fig. 196). Lastly, it must suffice to mention those glandular surfaces which 

 secrete nectar, of which examples are seen in the Bracken at the base of 

 each pinna (Fig. 195). AH of these, however biologically important in the 

 individual case, are but sporadic occurrences. 



It thus appears that the dermal and other non-vascular tissues of the 

 sporophyte in Ferns are generally of so variable a nature that they %\-'^& but 

 doubtful material for phyletic comparison. They stand in that respect far 

 behind the vascular system, which shows such a high degree of constancy 

 that it gives a relatively secur.e ground for comparative argument. Reserva- 

 tion must, however, be made in the case of hairs and scales. The view that 

 the simple hair is primitive and the expanded scale derivative follows partly 

 from the evidence of individual development, but much more securely from 

 the fact that simple hairs are present often exclusively in Ferns whose other 

 features, as well as their fossil relatives, stamp them as primitive. It will be 

 found that this is perhaps the most trustworthy criterion derived from tissues 

 other than the vascular. In fact it may be stated generally and with con- 



