3IO THE EMBRYO [CH. 



growing apex of the axis. The actual pole or centre of the stem-apex is 

 indicated by the cell-cleavages. It may be located accurately where there 

 is a single apical cell, as there is in the Leptosporangiate Ferns, or in 

 Equisetuni. After the epibasal hemisphere is defined by the basal wall {b, b), 

 the segments which follow it establish at once the initial cell at the point 

 nearest to its centre that is possible by the method of successive segmentation 

 (Figs. 294, a-c\ In these cases it is not going too far to say that the epibasal 

 hemisphere itself is the original initial cell for the shoot. In the Marattiaceae, 

 and others where the apical point is less exactly defined by segmentation, 

 its position is still the same. Accordingly the conception of the young 

 sporophyte as a primordial shoot of spindle-construction may be accepted 

 as general for the Filicales, and it applies also for other Pteridophytes(267). 



The next step will be to trace the relations of the other parts of the 

 embryo to the primitive spindle thus defined. They are, the first leaf, the 

 first root, and the haustorial foot. Of these parts, the root and foot appear 

 to be related to the central spindle according to convenience. Neither time 

 of origin, position, nor even their existence is fixed or immutable, but variable; 

 and especially in those forms which are recognised as relatively primitive. 

 This is the general position for Vascular Plants at large, and within limits 

 it is illustrated among the Filicales. 



Taking first the haiistoriwn or foot, it can hardly be defined as a 

 morphological entity. It originates from the hypobasal hemisphere of 

 the embryo. Where a suspensor is present, and the embryo is curved in its 

 development, the hypobasal hemisphere enlarges on its convex side. This 

 is seen especially in Lycopods ; and it appears also in Helmijithosiachys, 

 and the swelling is described as a "foot." \\\ Botrychium obliqnum,\vo\N- 

 ever, no definite foot is found. Where the suspensor is small or absent, 

 the greater part or even the whole of the product of the hypobasal hemi- 

 sphere may develop as a haustorial organ. According to circumstances it 

 may be symmetrical or lop-sided ; as for instance, in Ophioglossum vulgatum, 

 and Botrychium Lunaria and virginiammt (and also in Isoetes). In these 

 plants the first root appears to be epibasal in origin, and the whole hypobasal 

 region is recognised as " foot." In Salvinia where no root is present it is the 

 same. On the other hand, a definite foot is not recognisable in the Marat- 

 tiaceae (Fig. 286). Here the root, which according to Campbell originates 

 late, and apparently from about the limit between the epibasal and hypobasal 

 regions, grows directly downwards and approximately through the centre 

 of the hypobasal hemisphere. A foot as such is never organised. According 

 to von Goebel (254, p. 992), the cotyledon in these Ferns acts at first as an 

 haustorium, but it finally breaks through the upper surface of the prothallus, 

 and this he regards as an explanation of the absence of a definitely organised 

 foot. As von Goebel justly remarks (254, p. 979), " We can only speak of a 



