638 GENERAL COMPARISON OF THE FILICALES 



so as to follow roughly the probable phyletic sequence, the massive 

 sporangium of the Marattiaceae has its archesporium deeply sunk : the 

 walls all cut at right angles, and since the outer surface is but slightly 

 convex, the walls are almost parallel, and the archesporial cell approximately 

 cubical (Fig. 349 g). The segmentation in the Osmundaceae is variable, 

 and it has been observed to be so even in sporangia on the same plant 

 in Todea barbara. In some cases the archesporium is still square-based, 

 and may be square in its transverse section : but as the outer surface 

 becomes early convex, the lateral walls converge, and the archesporium 

 has the form of a four-sided truncated pyramid (Fig. 349 /). In other 

 sporangia of the same plant the lateral walls limiting the archesporium 

 converge more strongly, the outer surface being more convex, and one of 

 them inserts itself upon another : consequently the archesporium takes the 

 form of a three-sided pyramid (Fig. 349 e). It is to be noted that in 

 this figure the wall (x, x) is inserted on an inner periclinal but in 

 Fig. 349 d> which represents the segmentation in Schizaea, or in Tricho- 

 manes or Thyrsopteris, the wall (x, x) cuts another anticlinal. This marks 

 another step in attenuation of the sporangium, though only a slight one, 

 and in other essentials the segmentation is as in the simplest of the 

 sporangia of Osmunda or Todea. Figs. 349 b, c, show the segmentation 

 seen in various Gradatae : (c) corresponds to the condition of Alsophila 

 and Cyathea, and (b) is a slight variant upon it which is sometimes found : 

 it is seen also in Ctratopteris. In the Polypodiaceae, however, where the 

 sporangium may often be long-stalked, the wall cut by the wall (x, x) may 

 be no longer inclined, but transverse. From this series of diagrams it is 

 seen how gradual are the steps from the segmentation typical of the 

 Eusporangiate Fern to that of the most advanced Leptosporangiate. The 

 unity of the scheme cannot naturally be divided by any distinction of 

 origin from a single cell or from more. The difference of type thus gently 

 graded over is an index of the progressive attenuation of the sporangium 

 seen in descent, and it will be shown to go along with progressive reduction 

 of the individual productiveness. 



Closely related to the segmentation of the young sporangium is the 

 structure of its stalk when mature. Putting synangia aside, the stalk 

 varies from the short massive type of Angiopteris^ through various types 

 such as the Osmundaceae, Gleicheniaceae, Schizaeaceae, and Hymeno- 

 phyllaceae with relatively thick stalks, to the Polypodiaceae, where the 

 stalk is commonly attenuated and long. It may, in extreme cases, be 

 reduced to a unicellular filament, as in Scolopendrium. These steps again 

 show a general parallelism with the individual spore-output, the thickness 

 of the stalk being roughly proportional to the stream of nourishment 

 required. 



But it is the sporangial head, with its relatively thin wall surrounding 

 the cavity filled with spores, that is the most distinctive part, and as the 

 character of the opening mechanism, or annulus, has been made the chief 



