XXIV] SORUS 203 



dictyostelic. Moreover the ontogeny of D. pectinata points to a similar 

 conclusion 



Spore-producing Members 



The naked sorus of the Gleicheniaceae consists of a low circular receptacle 

 bearing a variable number of sporangia, which are quite separate from one 

 another normally, though instances of synangia which resemble a fusion of 

 two sporangia occur occasionally. The sporangia usually form a single row 

 round the receptacle, and their orientation in this case is constant, the 

 longitudinal slit of dehiscence facing directly towards the centre of the 

 rosette-like sorus. Gleichenia flabellata may be held as a central type of 

 the genus (Fig. 476). It has two parallel rows of leaf-segments of the 

 Pecopteris-type so common among Palaeozoic Ferns, and each segment 

 bears a row of sori on either side of its midrib. The number of sporangia in 

 each sorus is commonly 4 to 6. From such central numbers deviations 

 may be found within the genus, on the one hand to fewer sporangia, on the 

 other to a greater number. The former is seen in G. flabellata itself, for the 

 distal sori may bear three, two, or even only one: these are evidently results 

 of reduction below the normal, and are probably consequent on defective 

 nutrition. But in Eu-Gleichenia the number is commonly 3 to 4, while in 

 G. dicarpa two is the usual number. These species all have small rounded, 

 usually concave pinnules, each bearing a single sorus on the lowest acroscopic 

 vein: their sporangia are commonly protected by "ferrugineous wool," or 

 sometimes by "paleaceous hairs." They inhabit sub-tropical or temperate 

 regions, sometimes at high levels {G. dicarpa). The conclusion is that these 

 are exiguous, xerophytic members of a tropical family, with sori reduced in 

 number, and containing fewer sporangia than the central types; and that 

 protection is afforded by the concave pinnules, fluffy hairs, and even by 

 scales. These species are far from deserving the title of "Eu-Gleichenia" 

 being apparently derivative, and specialised in relation to climate. The 

 same probably holds for Stromatopteris and Platyzoma, both of which are 

 clearly xerophytic, and also show reduction in number of sori and of the 

 sporangia composing them. These reduced types have secured themselves 

 by their concave pinnules, and protective scales and hairs. 



On the other hand there are species included in \Dicranopteris which 

 diverge from the G. flabellata-iy^e of sorus both in the number and in the 

 position of the sporangia. This is so in G. linearis (Burm.) Bedd. (= G. 

 dichotoma, Willd.), a large and variable species, probably the commonest of 

 the genus, spread through the tropics of both hemispheres. The number of 

 capsules may here be 6 to 10 (Fig. 486). These are arranged as in the 

 radiate-uniseriate type: but the centre of the sorus, vacant elsewhere, is 

 often occupied by one or more sporangia (Fig. 487: compare also Fig. 489). 



