208 SYMMETRY OF THE SPOROPHYTE 



In the Gymnosperms the radial type is constant as a whole for the 

 cones, both male and female ; but, as in Welwitschia^ there may be a 

 more or less marked zygomorphy in the individual flowers. 



In the strobiloid Pteridophytes also the strobili are for the most 

 part of radial construction. In the Equisetineae this is the case with- 

 out exception ; also in the Sphenophylleae, so far as they are known. 

 The related Psilotaceae are also radial in their fertile region, with the 

 exception of the pendent species Ps. complanatum, Sw., which is isobi- 

 lateral, probably as a secondary modification in accordance with its pendent 

 habit : it bears its leaves and synangia only on the margins of the 

 flattened branches. In the genus Lycopodium the strobili are always 

 radial, even where the vegetative shoot is strongly dorsiventral. But 

 in Selaginella, though the large majority of species show a radial strobilus, 

 even where the vegetative shoot is dorsiventral, still the subgenera 

 " Homostachys " and " Heterostachys " have dorsiventral strobili. The 

 conclusion that these are derivative seems in this case unavoidable. 

 Isoetes, and the fossil Lycopodinous genera have all radial strobili. Even 

 Ophioglossum and Botrychium may be held to be of the same type, 

 their upright axis in the polyphyllous species bearing leaves of equal 

 size on all sides ; but in the monophyllous species only one of these 

 is as a rule expanded at one time; still, excluding the factor of time, 

 the type of arrangement is radial of the shoot as a whole. Helminthostachys, 

 however, is dorsiventral as regards the whole shoot; and this may well 

 be held, on comparison with the other genera, to be a derivative 

 condition. 



The case of the Ferns, including the Hydropterideae, is not so readily 

 brought into line with the strobiloid forms, owing to the sori and 

 sporangia being distributed widely over their large leaf-surfaces and 

 margins, while the sporophylls frequently show no differentiation in form 

 or position from the foliage leaves. Their condition will be more fully 

 considered later; meanwhile it may be pointed out that, within definite 

 circles of affinity, a radial conformation is seen in upright forms, similar 

 to that in strobili of other Pteridophytes, while those with oblique or 

 horizontal axis show dorsiventrality, such as is seen in the strobili of 

 certain Selaginellas and in Helminthostachys. Thus, though no definite 

 strobilus is present, the relations of symmetry of the fertile shoot in 

 Ferns are still comparable with those in other Pteridophytes. 



Thus a review of the strobili of Pteridophytes as a whole leads to 

 the recognition of a very great prevalence of the radial symmetry in 

 them. In several large groups, which are certainly primitive in character, 

 being represented early in palaeophytological history, the radial type of 

 strobilus is never departed from : in others only occasionally ; and this 

 radial character of the strobilus may even persist in cases where the 

 vegetative system shows dorsiventrality. 



An examination of vegetative shoots of vascular plants at large shows 



