XXVIII] SPORANGIA 255 



in size as the pinna-traces pass off from its margins: higher up the hooks 

 disappear, and distally it contracts to an oval form with a single protoxylem 

 (Fig. 523). Loxsomopsis has essentially the same stelar structure on a smaller 

 scale: these facts strengthen the comparison with Loxsoi/ia, and indicate ana- 

 tomically a primitive position for both. 



The ontogenetic origin of this solenostele was traced, at my suggestion, 

 by Dr McLean Thompson {Trans. Roy. Soc. Edin. Vol. lii, p. 728, 1920). 

 It was found to correspond in essentials though not in detail to that of 

 Gleichenia pectinata as regards the primary medullation and the formation 

 of an internal phloem (see Vol. i, Fig. 134). The further steps to complete 

 solenostely are suggested by the diagram, Vol. i. Fig. 135, p. 145. The 

 facts have already been discussed there in relation to the theory of the stele. 



SoRUS AND Sporangia 



The chief interest of these Ferns centres in the sorus. A vertical section 

 through that of Loxsoma when young is seen in Fig. 521, E. The cup-like 

 indusium encloses the short receptacle, the sporangia, and the hairs that 

 accompany them : there is an obvious basipetal sequence of the sporangia 

 (Fig. 521, i)'). The orientation of the sporangia is constant. They are pear- 

 shaped, and rise obliquely upwards, each having a complete annulus {C, D), 

 but the induration is very unequal; the cells of the distal half are larger and 

 fully thickened, the lower half consists of smaller thin-walled cells, hardly 

 differing from those of the rest of the wall except in form and arrangement: 

 it has evidently ceased to be functional. When mature a longitudinal slit 

 appears in a median plane, dividing the indurated group into two equal 

 halves {D). There is no differentiation of a stomium. The annulus straightens 

 on either side of the slit, and may even become reflexed, gaping like the 

 covers of an open book (see Vol. I, Fig. 206, also Hooker, Genera Filicnni, 

 PL 15). The shedding of the spores is on the principle of the inverted 

 pepper-pot. 



This anomalous sporangium of Loxsoma suggested earlier a comparison 

 with Gleichenia on the ground of the median dehiscence. But the facts for 

 Loxsomopsis lead to a different interpretation. Here the plan of the sorus is 

 the same. The sporangia are pear-shaped as before, and each has a complete 

 oblique annulus. But it is indurated all round, except for a lateral stomium, 

 well diflerentiated. A noteworthy feature is that the stomium may be 

 either on the right or left side of the sporangium (compare Fig. 524, A, B, C). 

 This instability of position of the stomium in Loxsomopsis, right or left, 

 suggests that the annulus is not strictly standardised, i.e. that various parts 

 of the ring may be thin-walled, or may be indurated. This has its bearing 

 on the question of the swing of the stomium from a median position as in 

 the Simplices, to a lateral position as in most Gradatae. The stalk in both 



