202 SYMMETRY OF THE SPOROPHYTE 



These types of symmetry are not restricted to any of the great groups 

 of plants : examples of any one of them may be found in any of the 

 great divisions of plant-life. But nevertheless, in certain circles of affinity, 

 one or other type of symmetry may be prevalent : thus in the Red and 

 Brown Algae the bilateral symmetry is common : among the sporogonia 

 of Bryophytes the radial construction prevails : the gametophyte of Liver- 

 worts is with very few exceptions dorsiventral. 



The further fact that a single shoot may be at first of one type, and 

 subsequently change to another type of symmetry, demonstrates that they 

 pass one into another. It can be shown both by comparison and by 

 experiment that this occurs within certain limits. The most frequent 

 transmutation is that from the radial to the dorsiventral, a change which 

 is of special importance in its bearings on the morphology of the 

 sporophyte. 



In discussing the subject of symmetry, it has hitherto been usual to 

 draw illustrations indifferently, either from the gametophyte or from the 

 sporophyte generation. Doubtless, in considering the phenomena of form 

 in their general aspects this is right : the wider the net is cast over the 

 area of fact, the greater the probability of arriving at a sound conclusion 

 as to the qualities and the causes of the several types of symmetry in the 

 Plant-body. But it is a different question to enquire into the effect 

 which modification of symmetry may have exercised in the evolution of 

 the neutral generation. Analogy, with corresponding phenomena in the 

 gametophyte, may assist indirectly : but in the elucidation of the actual 

 historical record these can only have a theoretical interest. According 

 to an antithetic theory the starting-point of the two generations has been 

 quite separate and distinct, and this must have its effect on the study 

 of their symmetry. 



In the case of the gametophyte various types of symmetry are found 

 in the plants of the present day : and since there is no reason to believe 

 that there was any common origin of all gametophytes from any one body 

 of definite form, there is wide room for speculation as to the source of 

 their varying form, and little hope of finality of conclusion. But in the 

 case of the sporophyte it is different : the ovum, produced within the venter 

 of the archegonium, is normally the starting-point for the sporophyte 

 generation in the Archegoniatae : in these plants it is approximately 

 spherical in form, and the conclusion follows, on comparative grounds, 

 that the initial form of the sporophyte was approximately the sphere a 

 body without polarity and of radial construction. The question to be 

 discussed in this chapter is, then, what modifications of forms this simple 

 body undergoes in the course of its development into the complex sporo- 

 phyte, as seen in Archegoniate Plants; and under what circumstances 

 those modifications may have been introduced. 



The development might, in the first instance, consist of simple 

 enlargement, together with cell-division, with or without a differentiation 



