EMBRYOLOGY 



469 



peculiarities he describes as other than extreme modifications along the 

 lines already indicated by the less specialised embryos of the family. 1 



The first detailed description of the embryo in any of the species of 

 Botrychium was given by Jeffrey for B. virginianum, and it has been 

 verified in many points by Campbell. The very large prothallus bears 

 its archegonia on the upper surface : after fertilisation the zygote enlarges, 

 and divides first vertically to the axis of the archegonium, and in the 

 hypobasal and epibasal hemi- 

 spheres the usual octant 

 divisions appear ; but these 

 segmentations are obscured 

 by the less regular divisions 

 which follow. The embryo 

 thus appears as an ellipsoid 

 body, in which no apical 

 cells are at first defined. 

 Jeffrey states that the whole 

 hypobasal hemisphere goes 

 to form the foot, while the 

 stem-apex and the root 

 originate from the epibasal 

 half: and his drawings cer- 

 tainly seem to bear this out 

 (Fig. 261). The apical cell 

 of the stem (a) is defined 

 before the cotyledon appears : 

 this is formed on the side 

 of the axis next to the root 

 (/>), and Jeffrey records that 

 it is derived from the shoot- 



meristem. It grows rapidly, and finally becomes expanded above ground 

 as the first assimilating leaf. The root is, however, the first part of the 

 embryo to emerge, and a second and third root may make their appearance 

 before the cotyledon unfolds : subsequently successive spirally arranged 

 leaves are formed on the 'axis, but the earliest fertile spike observed in 

 this species was borne on the ninth leaf. 



1 This interpretation of the data of Campbell differs widely from his own. It is 

 impossible here to enter into any full discussion of the question. It should be stated, 

 however, that Campbell's own view is that the type of embryo of 0. moluccanum is 

 probably the most primitive, and shows an embryo in which no axis exists at first ; he 

 regards the definitive sporophyte as a secondary structure developed as a bud upon the 

 primary root. In O. pendulum, also, the leafy sporophyte is secondary, neither stem-apex 

 nor leaf being produced from the embryo itself (I.e., p. 183). In fact, Campbell takes 

 as the most primitive forms those which are most divergent from the type of embryo 

 which is usual in other Pteridophytes. It would seem more satisfactory, however, in so 

 specialised a case as this, to start from the least divergent, such as 0. vulgatum. 



FIG. 262. 



rychiitm Lunaria, L. 36 = fertilised archegonium; 37 = 

 :, showing the first segmentation ; 38 = embryo of four cells ; 



Botrychi 



zygote, sho 



39-40 embryos cut in direction of the axis of the archegonium ; 

 42 = an embryo breaking out of the prothallus; 36-40X225; 

 42X150. (After Bruchmann.) 



