216 SYMMETRY OF THE SPOROPHYTE 
the embryo settles down at once into an upright radial type of structure: 
in others, and particularly in Z. 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 Z. cermuum 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. 
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 infer 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,? 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 
1The more exact comparison of the embryology in the genus Zycogodium will be 
taken up in the special part of this work. 
2«« Equisétales et Sphénophyllales. Leur origine filicinéenne commune.” Sud/. Soc. 
Linn, de Normandie, 1903, p. 93. A somewhat similar speculation has recently been 
published by Tansley (Mew Phytologist, 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” (/.c., p. 32). It appears 
preferable to adhere to observed facts, 
