382 EQUISETALES 



arose at an early date), the origin of the two bodies is alike; but the 

 sporangiophore, which is the more bulky, soon adopts a mode of growth 

 which leads to a shield-like form. He concludes that the simpler develop- 

 ment of the sterile leaf was the more primitive type, and that the stronger 

 growth of the lower surface of the sporangiophore, so as to give it the 

 hypo-peltate form, is a new development. He also alludes to the transi- 

 tional forms between the two types, such as have been described by Gliick 

 and others. 1 



Before the homology of the leaf-teeth with the sporangiophores is 

 accepted, the grounds upon which it is based are to be examined ; they 

 appear to be these : 



(1) Similarity of the cell-structure on first origin. 



(2) Similarity of position relatively to the axis. 



(3) Transitions, through the annulus and its malformations, from the 



one type to the other. 



The similarity of structure of the two as shown in vertical sections 

 was pointed out by Gliick, though, as he himself remarks (p. 362), it 

 holds only for the very earliest stages. But similarity of segmentation 

 has long ago been shown to be no proof of morphological identity in the 

 case of embryos and hairs ; without going so far afield as this, a com- 

 parison of a vertical section through the leaf-margin of Angiopteris? with 

 a vertical section through its sorus, 3 shows a near similarity of the 

 cell-net : yet this does not suggest any homology of the leaf-margin with 

 the lip of the sorus ; and no more can the similarity of segmentation at 

 the outset of that of the bract be held to prove the foliar nature of the 

 sporangiophore. 



Both sporangiophores and sterile leaves are lateral appendages of the 

 axis, but this does not of itself prove the point ; for instance, in plants 

 which bear prickles, the prickles and the leaves occur together on the 

 shoot ; and the former arise not much later than the latter, while similar 

 tissues take part in the formation of both. If both arose simultaneously 

 close to the apex, the early distinction of them would be a matter of 

 difficulty, though they are parts of different morphological character. It is 

 possible thus to contemplate the origin of parts of similar cellular structure, 

 but not morphologically comparable with one another, laterally upon the 

 same axis. 



The occurrence of middle forms between the teeth of the normal 

 annulus and sporangiophores appears at first sight important evidence ; but, 

 as is well known, intermediate forms occur between ovules and "foliage 

 leaves, and, nevertheless, the opinion is widely, and in my view rightly, 



1 Gliick, "Die Sporophyll-Metamorphose," Flora, vol. Ixxx., 1895, p. 364, and Plate 5. 

 References are there given also to Milde and other writers. 



2 Annals of Botany, vol. iii., Plate 23, Fig. 71. 



3 Phil. Tram., B, 1897, Plate 10, Fig. 66. 





