THE EMBRYO 



135 



II. THE EMBRYO. 



The study of" the embryogeny of the Marattiaceae offers many difficulties. 

 Fertilization does not seem to be of common occurrence and a very large number of 

 prothallia must be examined before even a small series of embryos can be secured; 

 moreover, the earlier stages of the embryo are peculiarly liable to shrinkage in 

 preparing them for sectioning and it is exceedingly difficult to secure really satis- 

 factory preparations of these early stages. 



A marked peculiarity of the young sporophyte, which was first shown by Luers- 

 sen and Jonkmann for Marattia and Angwpteris, is the orientation of the primary 

 organs of the young sporophyte with reference to the archegonium. The primary 

 or basal wall in the embryo is always transverse, as it usually is in Ophioglossutn, 

 instead of being vertical as in most of the typical ferns. The first leaf, instead of 

 being formed from the portion of the embryo nearest the archegonium, as it is in 

 the common ferns, arises from the half of the embryo which is turned away from the 

 archegonium, and grows straight upward, bursting through the prothallium upon 

 its upper surface, instead of appearing upon the lower side of the prothallium and 

 curving upward. The external organs of the young sporophyte are differentiated 



cot 



I'x.. 106. Marattia douglasii. 



A. Two longitudinal sections of a young embryo, b b, basal wall. X200. 



B. A similar section of an older embryo, cot, cotyledon; r, root initial. 



C. Three transverse sections of a much older embryo, showing junction of the two first leaf traces. X50. 



much later than is the cast- in the Leptosporangiates, and the Marattiacea? in this 

 respect closely approach the ( )phioglossacea?. 



It has been generally assumed that, as in the leptosporangiate ferns, the cotyle- 

 don and stem are of epibasal origin, the root and foot hypobasal. This conclusion 

 was reached by both Jonkmann and Farmer, and my early studies on Marattia 

 douglasii led me to the same conclusion. A further study of this species, however, 

 as well as an examination of the embryos of Angiopteris, Kaitlfiis.ua, and Datura, 

 has shown that this is not the case, but that the whole of the hypobasal region is 

 devoted to the formation of the foot, and the root is developed secondarily from the 

 epibasal region, from which are also derived the stem apex and the cotyledon. 



While in the typical ferns the young organs of the embryo at a very early stage 

 show a definite apical growth, the apical cells being readily traceable to the primary 

 octants of the embryo, this is by no means so readily shown in the Marattiacea?. 

 In the later stages, such initial cells can be seen in the root and stem at least, but the 

 relation of these initial cells of the root and stem to the early divisions of the embryo 

 is very difficult to determine. In M. douglasii (Campbell 3) I stated that there 



