OF ARTS AND SCIENCES. 71 



direct continuation of the prothallus cells, and not a distinct organ- 

 ization 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 characteristics 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. 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 

 Fig. 10. In the mean while, there appears on the basis of the leaf, or 

 on what is now so far differentiated that it is evidently the leaf-stalk, a 

 bud, which very soon can, by means of the cell-cap on its end, be rec- 

 ognized as a root. This grows always in a direction the reverse of the 

 leaf; that is, backwards away from the concavity. After the appear- 

 ance of the root, a bud appears on the basis of the leaf-stalk, looking 

 towards the concavity, and from this grows the stem. As a rule, the 

 leaf is tolerably far advanced in its development before the root appeal's, 

 and the root invariably precedes the stem-bud. The terms forward and 

 backward with relation to the concavity of the prothallus are, of course, 

 inapplicable when the young plantlet is formed at or near the end of a 

 process of the character above described, when the leaf and root shoot 

 out ad libitum. In all cases a vascular bundle traverses the leaf and 

 root, and these are in connection with the vascular bundle of the pro- 

 thallus. 



If now we compare Figs. 11 and 14, we shall clearly see that the 

 cases we have been discussing differ widely from the ordinary cases of 

 embryonal growth. Fig. 11 represents a longitudinal section through 

 the spot where a young plantlet, such as we have described, shoots out 

 from the prothallus (jo, ^>,), b represents the leaf, r the root, and s the 

 stem-bud, which was cut a little to one side of the median line. Fig. 

 14, taken from Sachs's Lehrbuch der Botanik, represents a longi- 

 tudinal section of a prothallus and a normally developed embryo at- 

 tached, the whole not so strongly magnified as Fig, 11. First, at a 

 glance, 14 differs from 11 in the fact that the young plant in the latter 

 case is so intimately connected with the prothallus that one cannot 

 decide where the one begins and the other ends ; while, in the former, 

 it is perfectly easy to trace the outline of the young fern. Secondly, 

 we have in 14 a structure known as the foot, f, by which the develop- 

 ing fern is separated from the prothallus, — a structure of which we 

 find no equivalent in 11. Thirdly, the vascular bundle of the plantlet 

 is in direct connection with vessels which lie wholly in the prothallus. 



