THE NATURE OF ALTERNATION, ETC. 331 



Onoclea sensibilis. 1 Scolopendrium vulgare. 5 



Athyrium Filix-fiX'minaP- Pteris aquilinaS' 



Aspidium (Polystichutri) annulare? Trichomanes alatum? 

 Nephrodium Filix-mas.^ T. pyxidiferum? 



T. Kaulfussii? 



It is unnecessary to give details of the phenomenon in 

 the different species, but the general facts may be sum- 

 marised. The prothalloid growths have been found to 

 arise from the leaves or from young sporangia ; in a 

 number of cases their production is associated with a more 

 or less complete sporal arrest. When the growth does not 

 proceed from the sporangium it may occur from cells of the 

 placenta, from the leaf-margin or from superficial cells of 

 the leaf above the termination of a vascular bundle. Some- 

 times this commences while the leaves are still attached to 

 the plant and standing erect ; in other cases it has been 

 induced by laying portions of the frond on damp soil. 

 Special interest attaches to cases of apospory described 

 in Nephrodmm Filix-mas 10 and Scolopendrium vulgare u in 

 which the prothalli arose from or near the margin of the 

 early formed fronds of the sporophyte. Since fronds of this 

 age do not in these species bear sporangia, sporal arrest can 

 hardly have been a factor in the causation of the apospory. 

 In N. Filix-mas the young plant was apogamously produced 

 and the prothalli on its fronds were also apogamous. 



Attempts to induce apospory by laying portions of fronds 

 on damp moss have been made without success, 12 but indi- 

 cations of the effect of interference with normal conditions 

 of life in at least predisposing to apospory are not wanting. 

 Thus the prothalli of Scolopendrium, a number of which 

 produced aposporous plants, had been repeatedly subdi- 

 vided. 13 The Onoclea the aposporous growths formed upon 

 sporophylls which had been induced to assume the charac- 

 ters of the sterile fronds by the removal of the latter from 

 the plant. This may be regarded as an instance of ex- 

 perimental sporal arrest. 14 



Atkinson (1), (2). 2 Druery (1), (4). 3 Wollaston ; Bower (1). 



4 Druery (3), (4). 5 Ibid. (4) ; Lowe. 6 Farlow(2). 



7 Bower (5). * Ibid. (12). 9 Lowe. 10 Druery (3), (4), 



11 Lowe. 12 Bower (6). 13 Lowe. 14 Atkinson (2). 



