62 Bower.—Studies in the Phytogeny of the Filicates. VII. 
Ferns, taking in this respect a lower place than any of the Dicksonioid 
series. It is difficult to estimate the phyletic bearing of this fact. It may 
be taken as indicating that there is a certain degree of freedom in respect of 
vascular construction, and that its features do not necessarily march parallel 
with other characters. It is, however, a fact of interest that the Lindsay a 
condition is a phase passed through in the ontogeny of allied Ferns. This 
has been seen with special clearness in the case of Histiopteris incisa , 
where it may extend for a length of three internodes (Fig. 23, pp. 30, 31), 
Lindsaya may then be held to have stood still anatomically at an early stage 
of its ontogeny, a fact possibly related to its often climbing habit and re¬ 
stricted leaf-development, while others with more ample leaves progressed 
to solenostely and other complications. 
Most of the Pteris series are remarkable for the persistence with which 
they retain in the axis the continuity of their vascular tracts. But in some 
cases perforation becomes a feature. It has been noted by Gwynne- 
Vaughan in Dennstaedtia. It occurs occasionally in Lonchitis\ but it 
becomes a marked feature in Pteridium . In the leaf such interruptions of 
continuity claim attention. The leaf-trace of the simpler examples is 
undivided (Lindsaya, Paesia ), and it is the same for some of the largest 
(P . podophyllci). But in Lonchitis, P. cretica, &c., it comes off at once 
in the form of two equal straps, by reason of a median perforation. A more 
prominent case of this initial perforation of the leaf-trace is seen in Sacco- 
Ionia elcgans (Fig. 16); the disintegration occurs here at a lower level than 
in Cibotimn} A similar condition is seen in Acrostichum aureum , while in 
Pteridium also the trace comes off in several separate straps. Thus it is 
seen that ‘perforation’ either of the stele or of the leaf-trace may occur 
sporadically in this series. In this, as in the medullary developments, there 
is a recurrence of features already present among the Dicksonioids. 
The type of sorus in all the Pteris series is referable in origin to the 
isolated, marginal, gradate sorus of the Dicksonioids, with its basal indusium 
more or less definitely two-lipped. The nearest approach to this is seen in 
Saccoloma elcgans , 2 where the sori are still discrete on the vein-endings, the 
only step towards fusion being in the upper indusium. It has been shown 3 
to be marginal in origin. It is but a slight step from this state to what is 
seen in Lindsaya* where the fusion of both indusia and of the receptacle is 
often complete, while the peripheral vascular commissure links the successive 
receptacles into a continuous fusion-sorus. But various degrees of this 
fusion may be seen in individual cases, the study of which indicates how the 
fusion-sorus has arisen. The condition of Paesia and of Pteridium illustrates 
1 Land Flora, Fig. 331. 
2 Goebel, Flora, vol. cv, 1912, p. 47, Fig. 10. 
3 Studies, III. Ann. of Bot, vol. xxvii, PI. XXXIII, Fig. 16. 
4 l.c., PI. XXXIV, Figs. 20, 21. 
