ASEXUAL GROWTH FROM PROTHALLUS OF PTERIS CRETICA. 269 



between the four surface cells (two of which are seen at z) 

 and the scalariform ducts, I am compelled to regard the two 

 cases as having only accidentally such a superficial cell-con- 

 formation. 



So far the changes mentioned have taken place in the plane 

 of the prothallus itself. Now a change occurs which pro- 

 duces a growth in a direction perpendicular to the prothallus, 

 and this growth is easily distinguished from the usual embryo 

 growth. A swelling is seen, generally on the under surface 

 of the prothallus, shortly after the appearance of the scalari- 

 form duct. This swelling is situated on or very near the line 

 connecting the original duct and the nearest point of the 

 concavity. When there is a process, this swelling very often 

 appears near its extremity, as in PL X, fig. 2, b. When two 

 such swellings appear simultaneously, they are generally 

 situated side by side. In all cases, there is seen behind 

 the swelling the scalariform duct or ducts lying in the sub- 

 stance of the prothallus itself. It is impossible for me 

 to say in which cells of the prothallus this swelling or 

 outgrowth originates. Longitudinal sections show no change 

 by which the cells of the outgrowing portion — which is, 

 in this case, on the upper instead of the lower surface 

 of the prothallus as is more commonly the case — can be 

 distinguished from the cells which are to remain a portion of 

 the prothallus. From the not unfrequent appearance of a 

 bursting through the surface, it may perhaps be mferred that 

 the superficial cells take no part in the growth. Certainly 

 no particular mother-cell or cells seem to be the place of 

 origin of the new growth, but it seems to be a direct continua- 

 tion of the prothallus cells, and not a distinct organisation 

 temporarily attached to it, as is the case with an embryo 

 growth. This swelling, to which I have intentionally avoided 

 giving the name of bud, develops and shows all the character- 

 istics of a fern leaf, and is, in fact, not a stem, but a true 

 leaf. When it arises on the under surface of the prothallus, 

 this leaf grows forwards, curves round the border of the 

 concavity, and raises itself into the air, as in PI. X, fig. 4, b. 

 When two such swellings occur by the side of one another, 

 one generally grows from the upper, the other from the 

 under surface of the prothallus, as in PI. X, fig. 5. In the 

 meanwhile, there appears on the base of the leaf, or on 

 what is now so far diflferentiated that it is evidently the 

 leaf-stalk, a bud, which very soon can, by means of the cell- 

 cap on its end, be recognised as a root (fig. 4, r). This 

 grows always in a direction the reverse of the leaf; that 

 is, backwards aiway from the concavity. After the appear- 



