Manchester Memoirs, Vol. lix. (191 5), No. 3. 7 



phology of Stigmaria the root-bearing region of Isoetes is 

 not mentioned, while comparisons are drawn with the 

 rhizophores of Selaginella, the basal root-bearing swelling 

 of Selaginella spinulosa, and with such further removed 

 structures as the root-bearing region of PJiylloglossum and 

 the leafless rhizomes of Psilotnm. Considering the close 

 comparison always made between other features of 

 Lepidodendrece and Isoetes, it is clear that, if Williamson's 

 comparison of the root-bearing regions is well founded it 

 has more direct morphological significance than any of 

 the others suggested. 



The reason why this comparison has been abandoned 

 seems to be that all later work on Isoetes has followed 

 Hofmeister's interpretation in regarding the roots as 

 arranged in vertical lines on a stem, and their production 

 and the downward growth of the stock as due to the 

 secondary meristem. This is the view taken in the full 

 and important investigation of the three-lobed stock of 

 Isoetes hystrix by Scott and Hill. 10 In that paper, the 

 arrangement of the roots in vertical lines is described in 

 detail but their development from the meristem is not 

 dealt with. No comparison with the Lepidodendreae is 

 made by Scott and Hill as regards the root-bearing 

 region. 



After this brief survey of the history of opinion on 

 the morphology of the stock of Isoetes the main facts of 

 construction as shown in two-lobed plants of I. lacustris 

 may be re-examined, although the result of this is on the 

 whole simply to confirm Von Mohl's description. The 

 re-description will, however, bring some additional features 

 into prominence and group the facts for convenient con- 

 sideration. 



The external form of the two-lobed stock of /. lacustris 

 '" Annals of Botany \ XIV., 1900, p. 413. cf. especially pp. 430, 431. 



