514 FILICALES 



suggested, and structurally the sorus itself offers no difficulty (compare 

 Figs. 283 H and K). It is thus seen that the general type of sorus is 

 constant in the family : the chief differences lie in the mode of associa- 

 tion of the sporangia, and in the extent and fission of the sori. 



As the development of the individual sporangium has been found to 

 be essentially the same in the several genera, notwithstanding the difference 

 that exists between the synangial state and that where the sporangia are 

 separate, it will suffice to describe it for one only, and Angiopteris, with 

 which Archangiopteris shows close similarity, may be selected as being 

 the most familiar. 1 At an early stage the sporangia begin to project as 

 separate upgrowths; but it is impossible, from a study of superficial sections, 

 to detect any regular system of segmentation which is maintained in all 

 sporangia; a comparison of the four sporangia, shown in Fig. 284 A ii 

 surface view from above, discloses no regular sequence of segmentations 

 and the cell-groups which will develop into the sporangia appear con- 

 sequently ill-defined. Of the sporangia a, b, c, d, shown, that mark( 

 (b) is believed to be the most regular and usual type ; and the cell 

 shaded in it are evidently sister cells, derived from a single parent cell 

 which, as we shall see, gives rise to the central and essential part of 

 the sporangium ; we may call this, as in the other genera, the superficial 

 parent cell. If a section were taken along a line x x through such a 

 sporangium, after it had grown more convex, it would appear as Fig. 284 B, 

 in which the cells shaded are believed to correspond to those shaded 

 in Fig. 284 A. It becomes apparent from such sections as these that 

 a single cell, the central cell, had divided periclinally to form an inner 

 cell and a superficial one ; the former is the archesporium, and has in 

 Fig. 284 B already divided into two ; the superficial cell has also divided 

 repeatedly. Though we may thus select sections so as to represent a 

 reasonably regular and typical structure of the young sporangium, it is 

 clear, from Fig. 284 A, that what has been described is only a central 

 type, and as a matter of fact hardly any two sporangia show exactly the 

 same details of segmentation. As development proceeds, growth and 

 cell-division often continue with sufficient regularity to allow the genetic 

 grouping of the tissues to be clearly followed (Fig. 284 c). Meanwhile, 

 certain cells at the apex enlarge to form the crest-like annulus. The 

 relation of this to the main lines limiting the product of the superficial 

 parent cell is variable ; a common case is that shown in the figure, where 

 the middle line (x) coincides with the limit of the annulus ; but this is 

 by no means constant : from this point (x) downwards, on the central 

 side of the sporangium, the de'hiscence will take place. The sporogenous 

 group is now clearly defined by the character of its protoplasmic body, 

 and it forms a definite block of cells, referable to a single parent. Next 

 follows the change of the cells immediately surrounding the sporogenous 



1 Compare "Studies," iii., Phil. Trans., 1897, where more full details are given for 

 this and other genera. 



