xiii] SPOROGENOUS CELLS 257 



" developmental potentialities " may be variously realised under the influence of external 

 and internal stimuli. He figures to himself a primitive sporangium with the capacity for 

 the cells of its wall to develop an annulus according to the position of the sporangium ; 

 and he suggests that the ring, for instance oi Hyinenophyllum, never had any other position 

 than that which we now see. He would only admit that a swing of the annulus had taken 

 place if it were proved that the position and form of the sporangium had previously been 

 other than that now seen. 



This somewhat drastic demand for evidence of an actual phyletic swing of the annulus 

 seems to be adequately met by two lines of rejoinder, one of which von Goebel supplies 

 himself by the facts detailed on the next page of his book (p. 1181). The existence of a 

 non-functional side of the annulus oi Loxsoma^ clearly traceable by its cell-divisions, is in 

 itself evidence of greater stability of development of the annulus than von Goebel con- 

 templates in his primitive sporangium. So also is the presence of a non-functional annulus 

 recognised by Campbell in Pilularia, as above described. 



On the other hand the very natural Dicksonia-Dennstaedtia series appears to supply 

 very cogent evidence of a swing having taken place. In Dicksonia the annulus appears 

 as a complete oblique ring. But in Dennstaedtia apiifolia, notwithstanding the basipetal 

 sequence of the sporangia, the annulus is nearly vertical, though still the chain of its cells 

 is completed by contact close to the insertion of the stalk (Fig. 252, C). But in D.rubiginosa^ 

 in which the mixed- character of the sorus has been assumed (Fig. 252,^5), the annulus is 

 actually interrupted at the insertion of the stalk (Fig. 252, D). When it is remembered 

 that Dennstaedtia was for long included in the genus Dicksonia this series appears to 

 illustrate within very near affinity a swing of the annulus, and a final interruption of it 

 which runs parallel with other features in this progressive series. A second instance is 

 seen in the Dipterid series. The oblique annulus of Dipteris appears to be shifted by 

 gradual steps to the vertical in the more advanced members of the series, such as Cheiro- 

 pleiiria and Platycerinm (Fig. 251, B, C). These and other sequences are held to give 

 support to the view that a swing of the annulus from the transverse to the oblique, and 

 from the oblique to the vertical position, with final interruption at the insertion of the 

 stalk, has actually taken place. In particular the occurrence of rudimentary, non-functional 

 cells of the annulus appears to indicate that the annulus is something more than an im- 

 mediate response to the influence of external and internal stimuli ; probably it is a 

 thing impressed upon the organism, so that its features are subject to hereditary trans- 

 mission, but liable to modifications such as those involved in a phyletic swing from a 

 transverse to an oblique, and finally to a vertical position. 



Origin of the Sporogenous Cells 

 The tetrahedral cell within the young sporangium, from which by further 

 segmentation the tapetum and the spore-mother-cells are formed, as seen in 

 Dryopteris (Fig. 15, 3), has sometimes been called the archespormm. In 

 most Ferns all the spores, and frequently the tapetum also, may be referred 

 in origin to a single archesporial cell. But this is not always possible. 

 Exceptions have been found to occur, for instance in Marattia (Fig. 254), 

 and the case seems doubtful also in Ophioglossum, though many preparations 

 of it would accord with such an origin. It does not appear that any general 

 principle is disclosed by tracing the sporogenous tissue to its source, through 

 '■ a last analysis " of the segmentations which give rise to the sporogenous 

 B. 17 



