. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. 572 FILICALES of Gleichenia, it is plain that the sporangia are of the same type, as regards the position of the annulus, though differing in the details ; or the comparison might be extended to the Schizaeaceae on the one hand, or the Hymeno- phyllaceae on the other, as regards the position of the annulus. The longitudinal slit of dehiscence traverses the distal part of the annulus, following the .median plane of the sporangium, and may extend some distance down its peri- a b pheral side, so that it
. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. 572 FILICALES of Gleichenia, it is plain that the sporangia are of the same type, as regards the position of the annulus, though differing in the details ; or the comparison might be extended to the Schizaeaceae on the one hand, or the Hymeno- phyllaceae on the other, as regards the position of the annulus. The longitudinal slit of dehiscence traverses the distal part of the annulus, following the .median plane of the sporangium, and may extend some distance down its peri- a b pheral side, so that it faces outwards from the receptacle. The orientation of the spor- angia being strictly upon the Gleicheniaceous type, it ap- pears that the main difference is that, while maintaining the same position of the annulus, the slit gapes towards the peripheral face of the spor- angium rather than on the central side of it. There is no differentiation of a stomium, but the rupture occurs regu- larly at the distal end (x, Fig. 320 d). The portions of the annulus on either side of the slit straighten as they dry and curve outwards in the usual way; they may even become reflexed, tearing away irregularly from the rest of the wall, or carrying frag- ments of it outwards; in this state the two flaps may appear like the covers of an open book. As the induration stops short about half-way down the side, the general form of the sporangium is not altered by the dehiscence, so as to press upon or displace neighbouring sporangia ; in fact, no elbow-room is required, as in Gleichenia, and this is a distinct advantage in a sorus where sporangia are numerous ; in this we may perhaps see the rationale of the incomplete annulus. No sudden jerks of the annulus have been observed, nor would such jerks be very efficient, since the majority of the spores lie below the flaps of the annulus; the shedding of the spores seems to be mainly on the principle of the Fig. 320. Loxsoma Cunninghami, Br.
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