2I 4 



SYMMETRY OF THE SPOROPHYTE 



succeeding leaves may vary, as was already noted by Hofmeister. He 

 stated specifically 1 that "the similarity in the development of the different 

 species of Ferns does not extend beyond the formation of the rudiments 

 of the first frond, and of the first root." He then proceeded to show 

 that whereas in Pteris aijuilina the second frond originates on the side 

 of the axis opposite to the first, and distant from it by about half the 

 circumference of the stem, in Nephrodium (Aspidium] Filix mas it is 

 at a distance of about a third of the circumference 2 : " the third diverges 

 from the second, and the fourth again from the third at about 120 

 degrees to the right, so that the fourth stands vertically over the first." 

 Thus the arrangement of the leaves is radial from the first in the Male 

 Fern. The same radial character, with varying divergences, has been 



found in other Fern-seedlings : for 

 instance, in Nephrodium dilatatiim, 

 Aspleniiim marinum, Pteris, tremula, 

 Osmunda regalis, and Todea superba. 

 The leaf-divergence in such cases is 

 variable, but approximates to a half, 

 or one-third, and it is spiral from the 

 first, without dorsiventral tendency. 

 This shows that in plants which are 

 radial in the mature state the initial 

 condition of the recumbent embryo 

 does not impress dorsiventrality on the 

 seedling. In others, however, where the 



through prothaiiusrand embryo, partly diagram- mature plant is markedly dorsiventral, 



of^ixis! the dorsiventrality asserts itself early. 

 Among these the case of Polypodiiim 

 vulgare is very instructive, as showing 



that the dorsiventrality of the mature shoot is not a direct consequence 

 of the initial procumbent position. The initial embryogeny is as usual : 

 the second leaf of the embryo is obliquely on the same side of the 

 axis as the first, and as the subsequent leaves are also inserted alternately 

 and obliquely on that same side, and the growth of the axis is stronger 

 on that same side which faces away from the prothallus, the whole 

 shoot becomes inverted by a strong curvature upwards through the 

 apical indentation of the prothallus ; and thereafter it pursues its horizontal 

 course backwards over the top of the prothallus (Fig. no). It is interest- 

 ing to note as a consequence of this that the side of the axis initially 

 turned downwards in the embryo becomes the upper side of the dorsi- 

 ventral rhizome, owing to the inverting curve : thus the creeping posi- 

 tion of the permanent shoot is not merely a direct continuance of the 

 initial prone position of the embryo. Other examples might be quoted, 

 but this will suffice to show that the dorsiventrality of the mature shoot is 



} Higlur Cryptogramia, Mug. .<]., p. 208. '-'/,.<.. p. 227. 



KIG. no. 



Polypodium vulgare. x6. Median section 

 hrough prothallus, and embryo, partly diagram- 

 matic : showing one series only of the distichous 

 leaves /], / 3 , etc. ; /f=roots; n^ = 

 The hyponastic shoot becomes completely in- 

 verted, growing backwards over the prothallus. 



