xxiv] 



SORUS 



207 



In the more attenuated G. linearis the sporangium is from the first more 

 elongated, and its stalk less massive (Fig. 490, g). The cap-cell appears at 

 a time when the sporangial head is more clearly in advance of the adjoining 

 tissue, and the central cell is never actually immersed in the tissue of the 

 receptacle. In fact the development is as in Leptosporangiate Ferns. 

 Sometimes the central sporangia are absent {g): but when present they 

 arise simultaneously with the rest, and occupy from the first the central area 

 of the receptacle (//, /). Their presence cannot then be accounted for simply 

 by displacement due to pressure: it is to be ascribed rather to interpolation 



h 



Fig. 490. «, /', c, sori of Gleichenia Jiabellata. a, />, in vertical, f, in horizontal section. 

 cf, e, /, sporangia of G. drcinata, showing central cell and tapelum. g, h, i, sori of 

 G. dichotoma (linearis) : in g the centre is vacant, in ii and / young sporangia appear in 

 the vacant space. J, /c, sporangia of G. flahellata with spore-mother-cells formed, and 

 very numerous. (a-/x 200; y, ii x loo.) 



of accessory sporangia not normally present in the flahcUata-X.y^&. It is 

 important to follow these differences within the genus in detail, for they 

 appear to provide within a near circle of affinity some interesting transitions 

 between the Eusporangiate and the Leptosporangiate types of sporangial 

 formation. 



The divisions in the sporangial head follow in the main the Leptospor- 

 angiate type. The annul us soon makes its appearance, only part of it 

 originating from the cap-cell. The tapetum divides as usual into two layers, 

 of which the outer remains narrow and tabular, and traces of it may be 

 found in the mature sporangium. The cells of the inner layer enlarge 



