Manchester Memoirs, Vol. lix. (191 5), No. 8 47 



to the normal secondary xylem of the Lepidodendrese. 

 It certainly corresponds in position to this, although 

 almost entirely laid down in the procambial region. On 

 this view the stele of Isoetes would contain the tissues 

 corresponding to the centripetal primary xylem, the 

 normal secondary xylem and also to the anomalous 

 secondary tissues (found in some species) of Lepidoden- 

 dreae. The comparison cannot be followed further at 

 present, since it would involve entering into details regard- 

 ing disputed questions with regard to the extinct plants. 



Another interesting aspect of the stele of Isoetes may 

 lastly be touched upon as bearing on general stelar mor- 

 phology. The other existing Lycopodiales (Selaginella 

 and Lycopodiinri) contrast with the Filicales in having a 

 central column of centripetal xylem on to which the small 

 leaf-traces join without affecting the periphery of the 

 stelar xylem. In Lycopodium, however, Sinnott 13 has 

 shown that the stele is just mesarch when a leaf-trace 

 has joined it. In the stele of some of the Filicales I have 

 been led to attach importance to the distinction of inner 

 xylem (corresponding to the centripetal xylem of Lyco- 

 podium) and outer xylem ; 14 the latter, together with the 

 bases of the leaf-traces forms a peripheral zone of xylem. 

 Isoetes alone, among existing Lycopodiales, exhibits a 

 corresponding state of affairs, and this may perhaps be 

 associated with the fact that it is a relatively large-leaved 

 form At any rate, the outer xylem of Isoetes corresponds 

 on the one hand with the outer primary xylem of the 

 Filicales, and on the other hand has been seen to corre- 

 spond in morphological position to the secondary xylem 

 of the Lepidodendrese. The Ophioglossacea; show, how- 

 ever, that a sharp line cannot be drawn between outer 



18 Bot. Gazette, 48, p. 138. 

 >* Ann. Bot., XXIX., p. 37. 



