2i6 SYMMETRY OF THE SPOROPHYTE 



the embryo settles down at once into an upright radial type of structure : 

 in others, and particularly in L. cernuum, which has been made the subject 

 of special study and comparison, the embryo may show at first a marked 

 dorsiventrality ; but it is at the same time exceedingly variable in form, 

 and in some individual cases the embryo of L. cernuum may closely 

 resemble the ordinary radial type of other species. This variability will 

 in itself discount arguments based upon details of form, and suggests 

 that the dorsiventrality where it occurs is the result of relatively direct 

 adaptability of a very plastic organism. 1 



The facts and arguments brought forward in this chapter lead up to 

 a general view of the symmetry of the sporophyte generation. It would 

 appear probable that the original type of its construction has been radial 

 throughout, a condition which commonly goes along with a vertically 

 upright position. This is the position of the vast majority of Bryophyte 

 sporogonia : in them the radial construction is rarely departed from, and 

 where this does happen the dorsiventrality is readily referable to a 

 modification of a radial type. The greater diversity of habit of the 

 Pteridophytes, especially as regards the sporophyte, necessarily brings 

 greater difficulties in attaining to any general opinion for them ; but a 

 careful review of their various types, and especially a comparison of 

 members of the same group of them inter se, leads back constantly to 

 the radial type as primitive, even in cases where dorsiventrality is most 

 marked. The fact that in the Equisetales and Sphenophyllales the radial 

 construction is predominant, while it is also prevalent among the more 

 primitive Lycopodiales, and in a less degree in the Filicales, shows the 

 strong hold which the radial construction had among very early types. 

 In fact the position is fully strong enough to justify the general state- 

 ment that the radial mode of construction was primitive for the sporophyte 

 at large ; and that where dorsiventrality occurs, it is a secondary 

 condition. 



This conclusion is plainly out of harmony with the theoretical posi- 

 tion of Lignier, 2 who would refer the sporophyte as well as the gametophyte 

 to a hypothetical thalloid origin : this thallus, which was dichotomous, and 

 lay flat upon the soil, tended to curve upwards, and consequently to 



1 The more exact comparison of the embryology in the genus Lycopodiuin will be 

 taken up in the special part of this work. 



2 " Equisetales et Sphenophyllales. Leur origine filicineenne commune." Bull. Soc. 

 Linn, de Nonnandie, 1903, p. 93. A somewhat similar speculation has recently been 

 published by Tansley (New Fhytologist, 1907, p. 25) ; he refers the Archegoniatae in 

 origin to some " hypothetical Archegoniate Alga." He also passes lightly over the 

 transition from a sympodial rhizome to an upright, radially organised type (p. 33). It is 

 necessary, however, to remember that, as a matter of observation, all Archegoniate 

 sporophytes are initially of radial construction. The same difficulties appear to confront 

 both Tansley's and Lignier's hypotheses. To meet them both authors postulate hypo- 

 thetical forms which are "of course the purest speculation" (I.e., p. 32). It appears 

 preferable to adhere to observed facts. 



