3o8 



THE EMBRYO 



[CH. 



sporeling emerges on the lower side of the prothallus, its axis being prone, 

 and without any curvature within the thallus. Curvatures may, however, 

 appear later in the adolescent state, varying according to the adjustment of 

 the shoot for adult life. 



These facts relating to the primary embryology of the Filicales provide 

 material for a general opinion on the polarity and orientation of the embryo. 

 It appears probable that a suspensor was characteristic of primitive Ferns, 

 and that the primary construction of their embryo was filamentous. This 

 has been clearly stated by Lang, as follows (256 bis^ p. 35) : "The presence 

 of a suspensor of one or two tiers appears to be a fact of organisation in a 



Fig. 294. a = embryo o{ Equisetiim , after Sadebeck ; b = Marszlia,a.fter Hanstein ; 

 c = Adiaitticm, after Atkinson: all are orientated with the axis of stem and 

 root vertical, to which line the basal wall is variously inclined. 



d, e are diagrams to show in view from above (d) and in section {e) how a single 

 tetrahedral initial cell is established in the epibasal hemisphere. ^ = quadrant 

 wall; (9= octant walls. The cleavages thus initiated are continued as the 

 series i, ii, iii, iv. The result is that the initial cell (shaded) is formed at the 

 nearest possible point to the centre, consistent with the sequence of the seg- 

 mentations. 



number of forms which are relatively primitive. Its presence may be looked 

 upon as the last indication of the construction of the plant-body from a 

 filament or row of cells, i.e. as a juvenile stage in the development rapidly 

 passed over, and often suppressed." That it was liable to be suppressed both 

 in the Filicales and the Lycopodiales (and possibly also in the Equisetales) 

 accords well with the facts disclosed for these classes. Its survival may 

 perhaps have been determined by the advantage of thrusting the embryo 

 into the massive nutritive prothallus. But on the other hand the presence 

 of a suspensor appears to have fixed the orientation of the spindle-like 

 embryo so that where it is present the apex of the shoot is always directed 

 away from the neck of the archegonium ; it might be described as being 



