xxiv] 



SORUS 



205 



or less elongated, and its apex covered by numerous sporangia: and it is 

 important to note that the first steps are taken within the genus Gleichenia. 

 The sporangia of the Gleicheniaceae are pear-shaped with a very short 

 stalk, and an annulus consisting typically of a single row of cells: it is com- 

 plete round the sporangial head except at the line of dehiscence, which is 

 median and directed away from the lower side of the leaf (Fig. 489, i-n)_ 

 Its oblique course defines two thin-vv^alled areas: the central or basal which 

 faces obliquely to the centre of the sorus, and the [peripheral or distal which 



Fig. 488. rt, /;, Gleichiiiia fectiiiata Pr. a, shows the vcnalion and tlie mature sori, some of 

 which have dehisced. /', shows a single sorus, with its sporangia so closely packed as to be 

 flattened against one another, one has already dehisced, one sporangium on the right is 

 inverted. (Drawn by Mr Maxwell.) 



faces obliquely towards the leaf-surface. These correspond to the basal and 

 distal surfaces in the Schizaeaceae, both being constructed on essentially the 

 same plan. (See Vol. I, p. 254. Fig. 250.) 



There is considerable difference in size of the sporangia in the genus 

 Gleichenia. Those species which have a small number of sporangia in the 

 sorus, such as G. rupestris and circinata, have relatively large sporangia, 

 and their spores are large (Fig. 489, i,J, k) : those which have more numerous 

 sporangia have them of smaller size, e.g. G. linearis (Fig. 489, /, m, n). In 

 G. circinata the form is almost that of a kettle-drum: the stalk is short, and 



