26 Lang, Morphology of the Stock of Isoetes lacustris. 



The primary distinction of the body of Gymnosperms 

 and Angiosperms into main root and shoot is not strictly 

 comparable and may be left out of consideration with 

 the remark that it presents interesting analogies. In the 

 Pteridophyta the primary root of the embryo does not 

 develop, in any known case, into a main root producing 

 the whole root-system of the plant. In Ferns and Equise- 

 tum so-called adventitious roots are borne in ascending 

 order, either all along the shoot or limited to the lower 

 region. This is also the case in Lycopodium, but in the 

 Lycopodiales we also find plants with the roots confined 

 to the basal region. It is with these that Isoetes can be 

 compared in this respect and it is of interest to find that 

 the genera in question {Selaginella ^ Plearomeia, Lepido- 

 dendron, Sigillaria) are all plants with which it is pro- 

 bably more or less directly related ; this is shown by 

 the agreement in other morphological features. The 

 comparison with Selaginella is somewhat distant, though 

 interesting, the interest being increased by the existence of 

 additional rhizophores borne on the shoot. It is sufficient 

 here to mention the development of two roots in addition 

 to the primary root at the basal region of the young 

 plants of most species, and the more marked basal root- 

 bearing region of Selaginella spinulosa. In the latter all 

 the roots are produced in regular succession on the basal 

 region, the central vascular system of which is modified 

 in relation to the root insertions. 



The comparison with the root-bearing region of 

 Plenromeia or the Lepidodendreae is more direct, in view 

 of the possible phylogenetic connection between them 

 and Isoetes. In making it the peculiarities in the stock of 

 Isoetes due to the absence of growth in length must be 

 borne in mind and allowed for. If both shoot and 

 rhizophore of Isoetes be supposed to undergo sufficient 



