IX THE BRYOPHYTA 203 



of the products. Such a condition is seen in the spherical sporogonium 

 of Riccia, which has habitually been held to be primitive in its simple 

 characters of structure and form (compare Fig. 18). Here there is no 

 polarity : no distinction of apex and base. This character it shares with 

 the earlier .stages of some other embryos of Archegoniatae, which enlarge 

 at first as a simple sphere. But a distinction of apex and base soon 

 makes its appearance in all the more complex forms, with or without a 

 localised apical growth. The two great series of Archegoniatae differ 

 widely in the symmetry of their further development. The Bryophyta, with 

 very few exceptions, which will require special consideration, show polarity, 

 but retain their radial symmetry. Not a few of the Pteridophyta also 

 retain their radial symmetry, but under modifications which necessarily 

 follow as a consequence of their leafy habit : others, however, depart 

 broadly from it, some at an early period of their individual life, others 

 at later periods. 



The general view which is implied in the preceding paragraph is that 

 the radial type of symmetry is the prior condition for the sporophyte at 

 large. This opinion is not based merely on the fact that the ovum from 

 which all sporophytes spring is spherical. Much stronger grounds are to 

 be found, first, in the high degree of constancy of the radial type of con- 

 struction in the sporogonia of Bryophytes : while it is also frequent in 

 the Pteridophytes and Seed-Plants, especially in their strobili and flowers. 

 Secondly, in the fact that it is possible in many cases to refer the dorsi- 

 ventral symmetry, where it exists, to the unequal incidence of external 

 conditions, and to see by experiment how such conditions may bring 

 about some dorsiventral modification of a structure which is in the first 

 instance radial. Examples of this may be quoted occasionally from the 

 Bryophytes, and frequently from the Pteridophytes, and from the vegetative 

 shoots and flowers of Phanerogams. There is thus not only a compara- 

 tive, but also an experimental basis for the opinion that the radial 

 symmetry is the primitive, and the dorsiventral the derivative condition in 

 the sporophyte. 



Few facts relating to any large group of organisms are more 

 impressive than the constancy of the radial symmetry throughout the 

 sporogonia of Bryophytes. That body, originally spherical, becomes more 

 or less spindle-shaped in its later development, with or without a localised 

 apical growth. Zones higher or lower on the spindle-shaped body may 

 undergo more strong development than the rest, especially towards the 

 distal end, which is to be the fertile capsule. This is commonly seen, 

 both in Liverworts and in Mosses, but the Splachnaceae stand out as 

 extreme examples, and in Splachnum luteum the apophysis immediately 

 below the capsule is expanded into a wide disc (Fig. 102). Nevertheless, 

 here also the development is uniform all round in any transverse zone, 

 and accordingly the radial construction is accurately maintained. The 

 constancy thus usual for the sporogonium in itself directs attention to 



