Vol. 57.] AFFINITIES OF THE EH^TIC PLANT NAIADITA. 311 



sideration ; and the absence or non-preservation of cortical tissue 

 seems to be a feature in stem-structure sharply contrasting with that 

 typical of the Lycopodiacese. 



In connection with both of these characters it is important to 

 recognize the probability that Naiadita was a submerged species. 

 This Mr. Starkie Grardner has already maintained, on the evidence 

 of the associated freshwater fossils and that of the general habit 

 of the plant itself. On a priori grounds one might feel safe in 

 assuming that stomata would necessarily be absent in any submerged 

 plant : they are absent in subaquatic phanerogams, whether the 

 submerged habit be newly acquired or long established ; and that 

 submergence may produce the same effect upon a cryptogamous plant 

 is shewn by Jsoefes. Moreover, their normal function is bound up 

 with a subaerial life. Hence it was with no little surprise that in 

 examining the leaves of the only recent submerged Lycopodium 

 (namely, L. alopecuroides var. aquatlcum) I found unmistakable 

 stomata.^ The leaves of this variety consist of a single layer of fairly 

 stout-walled rectangular cells — the upper epidermis — and of one or 

 two layers of excessively thin-walled cells, the outermost of which 

 is the lower epidermis, in which the stomata are situated. The 

 actual existence of this structure in the leaf of a recent lycopod 

 strengthens the probability of the interpretation of the single layer 

 of leaf-cells in Naiadita as the only preserved representative of a 

 many-layered leaf. At the same time the situation of the stomata 

 in a delicate layer of cells, which would have escaped preservation, 

 affords an alternative explanation of the absence of stomata in 

 Naiadita. 



As regards the stem of this recent aquatic variety of Lycopodium^ 

 the cross-section of the cortex is identical with that of a recon- 

 struction which I had already drawn of the stem of Naiadita. 

 The arrangement of the tissues is that of Selaginella carried to 

 extreme. The whole cortical tissue from epidermis to stele is re- 

 placed by an air-space traversed by delicate trabeculae of thin-walled 

 cells. Curiously the preservation is not good enough to admit of an 

 attempt to reconstruct the stele. The most that can be said is that 

 it appears to have contained a band-like xylem-plate, the elements 

 of which had for the most part a wide lumen. 



If we may accept the affinities here suggested for Naiadita, 

 we have in this plant the earliest recorded instance of a fossil 

 member of the Lycopodiacese, resembling in proportions and out- 

 ward morphology the existing representatives of the group, but sepa- 

 rated from them by the whole extent of the Mesozoic and Tertiary 

 Epochs. 



^ A. Braun has pointed out that several submerged species of Isoetes bear 

 stomata on the leaves. See Sitzungsber. d. k. preuss. Akad. d. Wissensch. 1863, 

 p. 647 : quoted by Scott & Hill, Ann. Botan. vol. xiv (1900) p. 443. 



