OF ARTS AND SCIENCES. 69 



Others. This narrowness may have been only accidental and the result 

 of crowding in the pot, as very often happens in the cultivation of 

 ferns. In a single case, Fig. 13, one side was developed into a sort of 

 secondary prothallus. The cells of the prothalli were perhaps some- 

 what paler than usual ; and those which, near the concavity of the 

 heart, are generally more numerous than in other portions and isodia- 

 metrical, were here much longer than broad, — that is, longer in the 

 direction from the centre of the prothallus towards the concavity. As 

 is well known, fern prothalli are generally heart or kidney-shaped, 

 and the two sides composed of a single layer of polygonal cells, the 

 centre of a portion decidedly thicker and consisting of several, layers, 

 which we may call the cushion ; and in this last-named portion are 

 situated the archegonia, while the antheridia are much more widely 

 dispersed, being found also in the lateral lobes. As before said, the most 

 striking feature of the abnormal prothalli was the presence of a dotted 

 duct in the cushion a short distance back of the concavity, just where 

 the archegonia are generally found. But wherever such scalariform 

 vessels were present there were no traces whatever of archegonia to be 

 found, although antheridia were always abundant, as well as the hairs, 

 which here fulfil the offices of roots. See Figs. 1, 6, and 9, in which a 

 shows the position of the scalariform ducts. As may be seen from Figs. 

 6 and 9, the scalariform ducts arise singly, and are situated in the central 

 portion of the prothallus. They scarcely differ in shape at first from 

 the adjoining cells, wliich are longer and relatively narrower than the 

 superficial cells. The ducts increase by division in a direction parallel 

 to the surface, so that, in a longitudinal section, we find several lying 

 one above the other. 



Another peculiarity often, but not always, accompanying the presence 

 of scalariform ducts, was the formation of a process or outgrowth in the 

 concavity of the thallus, as shown in Figs. 1, 6, and 12. This out- 

 growth was variable in length, often being short and imbedded between 

 the lateral lobes, but sometimes projecting as a narrow tapering pro- 

 cess. In one case, it was forked at the extremity. The growth by 

 means of a single terminal cell is shown in Fig. 3. As just mentioned, 

 the existence of a process in the concavity is a striking peculiarity, but 

 not quite a constant occurrence, as is the presence of scalariform ducts. 

 The first scalariform duct arises in the prothallus, as I have just re- 

 marked ; and soon appear others, always in a line between the original 

 duct and the nearest point of the concavity. In this way arises an 

 interrupted row of ducts, which may extend, when a process is present, 

 nearly to its extremity. The cells surrounding the original vessel 



