CH. XXXVIII] PTERIDIUM 41 



explained later, the first choice will fall upon the Bracken Fern, Ptcridiiim. 

 Then will follow a comparison with such other genera as have always been 

 held to be closely allied with it, even though they may not always possess 

 the double indusium. In fact we shall first examine the Ferns included under 

 Diels' heading of the Pterideae-Pteridinae {I.e. p. 287), taking them approxi- 

 mately in the reverse order of their description by him. Those genera are 

 Pteridmm, Paesia, Lonchitis, Histiopteris, Pteris, Ochropteris, Anopteris, 

 Amphiblcstria, and with certain more doubtful congeners in Casscbecra and 

 Actiniopteris. The three first-named genera were grouped by Prantl as his 

 "Lonchitidinae." Of all the Fern-systematists of the 19th century Prantl 

 was the writer who most readily worked phyletic views into his classification, 

 the basis of his comparisons being widened by the results of his excellent 

 laboratory-technique. He himself regarded his Lonchitidinae as the most 

 primitive of the Pterideae, a position which accords with the reasoning to be 

 developed here; and this provisionally gives justification for describing them 

 first {Art. Konigl. Bot. Gart. Breslau, 1892, pp. 16-18). 



Pteridium 



The Common Bracken is one of the most successful of all vascular plants, 

 as shown by its cosmopolitan distribution and its gregarious habit. This has 

 probably been promoted by its underground creeping rhizome, which is 

 thus protected during adverse seasons. The unusual habit is shared by 

 Stroniatopteris, but without any similar cosmopolitan success (Vol. 11, p. 200). 

 The habit is established in the Bracken immediately on germination. After 

 bearing 7 to 9 alternating leaves the primary axis bifurcates, and each 

 shank burrows downwards into the soil bearing alternate leaves, and forking 

 repeatedly. The further development of the branch-system tends to become 

 dichopodial (Fig. 606): see Vol, l, pp. 74-76. As a progressive rotting of the 

 older parts reaches a branching and passes it, two separate individuals ma\- 

 result. It is to this method rather than to sexual propagation that increase 

 of individuals is mostly due. The long-stalked leaf bears a highly-branched 

 deltoid blade, with narrow pinnatifid segments having an open venation. 

 The rachis is marked by lateral pneumatophoric lines, which are continued 

 upwards to the pinnae. Glands secreting nectar are seated on the bases of 

 the lower pinnae, and are visited by ants (Sir F. Darwin, MS. 1876): but 

 their exact use is uncertain (see Vol. l, p. 205, Fig. 195). The rhizome and 

 young leaves are covered by a felt of simple hairs, and scales are absent. 

 The equal dichotomy, open venation, and the absence of protective scales 

 may be held as indications of a relatively primitive state. 



This conclusion appears to accord ill with the elaborate vascular structure 

 of the rhizome and leaf-stalk. The stelar system has already been described 



