344 ORGANOGRAPHIC COMPARISONS [CH. 



an archegonium is present in all the known primitive plants of the land. 

 How the archegonium originated, and the consequent encapsulation came 

 about, are still matters of surmise rather than of demonstration. An internal 

 embryology is a necessary consequence. But this woul-d itself restrict any 

 filamentous development of the embryo. Consequently, though the evidences 

 of this primitive form of construction of the sporophyte are held to be 

 a sufficient indication of its having been general, those evidences have in 

 many cases been almost eliminated. This is the interpretation here given 

 of the state of the embryo seen in all the more advanced Ferns, where no 

 suspensor is present. They are all held to have lost in the course of their 

 Descent that most obvious indication of their ultimate filamentous origin 

 (310,310. 



General Conclusion, and Morphological Comparison 

 with other plants 



A theory of the " Primitive Spindle " has recently been formulated for 

 plants at large(3i7). It brings into prominence that filamentous structure 

 which, though frequently disguised, is shown by a wide comparison to be 

 inherent in their embryology. Its application to the sporophyte of Ferns, 

 as based upon the comparisons and arguments contained in this volume, 

 may be stated thus : The structure of the sporophyte is essentially based 

 upon the primitive or, better perhaps, the primordial spindle, the polarity of 

 which is defined by the first segmentation of the zygote. The apex of the 

 spindle lies as near as possible to the centre of the epibasal hemisphere : its 

 base, as seen in certain primitive Ferns still living, is the tip of the suspensor. 

 The basal pole of it, known as the suspensor, is filamentous, and is regarded 

 as vestigial. It forms no permanent organ of the plant, and it is present 

 only as an embryonic structure in the more primitive types : in those which 

 are more advanced it may be entirely absent, presumably by abortion. It is 

 believed that in the course of Descent the spindle has developed dichopodially 

 at its apex to form the leafy shoot as it is seen in Ferns. Theoretically 

 the whole shoot of Ferns may be regarded as a specialised dichopodium, 

 which originated from equal dichotomy of the apex of the primitive spindle. 

 The shoot is developed primarily as a radial structure, but it is liable to 

 dorsiventral modification. One shank of the first distal forking of the spindle, 

 recognised in the embryo of a Fern as the axis, is endowed with slow but 

 continuous apical growth : the other, which grows more strongly, is the first 

 leaf, or cotyledon. The successive later leaves may also have originated in 

 Descent as shanks of such forkings of the apex repeated, and developed 

 dichopodially. But all the post-cotyledonary leaves are actually seen to arise 

 laterally below the apex of the axis, that is monopodially. This position 

 may, however, have been attained secondarily in Descent, just as the mono- 



