THE EMBRYO 53 



crescent, the horns of which unite at about the level of the stem apex, so that the 

 latter, together with the section of the second leaf, is completely inclosed by the 

 sheath formed at the base of the cotyledon. The leaf trace in both the cotyledon 

 and the second leaf is broadly oval in outline, increasing in breadth lower down, 

 so that the section there becomes crescent-shaped and the two together form an 

 imperfectly closed ring of small cells, the procambium of the future vascular cyl- 

 inder of the axis, which clearly is formed entirely from the united leaf traces. 



The apical cell of the stem appears triangular in cross-section, and from it are 

 cut off regular segments, but, as we have seen from the study of the longitudinal 

 section, the tissues which are developed from these segments below the apical cell 

 remain undifferentiated and form merely a mass of large parenchyma cells with 

 thin walls, which fill the space within the ring formed by the confluent leaf traces, 

 and thus constitute the pith or medulla of the stem. 



Jeffrey (Jeffrey 1, page 19) refers to the occurrence of short tracheids in the 

 prothallial tissue in one case which came under his observation. There was some 

 evidence that an embryo had been present, but it had disappeared, and he thought 

 it possible that this was a case of apogamy, similar to that which has been repeatedly 

 observed in Ptens cretica and in various other leptosporangiate ferns. If apogamy 

 does really exist in B. virgimanum it is the only case which is known among the 

 Eusporangiatse 



Botrychium obliquum (Lyon I) differs strikingly in the form of the embryo 

 from either B. virgimanum or B. lunaria, and these differences are so marked that 

 Lyon proposes to separate B. obliquum and a number of allied species as a new 

 genus, Sceptridium. The ovum in Botrychium obliquum becomes very much 

 elongated after fertilization, and it is probable that the first wall separates the part 

 next the archegonium from the inner cell, the former developing into a suspensor, 

 very much as we shall see that it does in Dancea, and this suspensor becomes well 

 developed and is conspicuous in the later stages of the embryo. Unfortunately, 

 Lyon gives no details of the further history of the embryo until the definitive organs 

 are far advanced. He states that the suspensor becomes elongated, pushing down 

 the rest of the embryo, which he calls the "protocorm," believing that it is homol- 

 ogous with the so-called "protocorm" of Lycopodium — a resemblance which would 

 probably disappear were a more thorough study of the subsequent history available. 

 This development of a suspensor is the first case described in the ferns, but, as we 

 shall see presently, a similar suspensor is regularly present in the genus Dancea, 

 which in other respects shows some interesting resemblances to B. obliquum in the 

 development of the embryo. Moreover, as we have seen, there is often an elonga- 

 tion of the upper cells of the young embryo of B. virgimanum, which suggests a 

 rudimentary suspensor. 



Lyon figures only a very advanced stage of the embryo, at about the time when 

 the root first emerges from the prothallium. At this time, aside from the presence 

 of the suspensor, the most marked difference between B. obliquum and B. virgin- 

 lanum is the apparent absence of the foot in the former species, while this organ is 

 so remarkably developed in B. virgimanum. The root in B . obliquum penetrates the 

 lower surface of the gametophyte, the cotyledon breaking through on the upper 

 side, so that the relative position of the leaf and root is like that of Ophioglossum 

 moluccanum, or the Marattiaceae. In the orientation of the root and shoot, there- 

 fore, B. obliquum is much more like Dancea or Ophioglossum moluccanum than it 

 is like B. virgimanum. 



It is to be hoped that further investigations will be made upon this extremely 

 interesting species. We anticipate that when the history of the earlier stages is 



