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Stomatal mechanism as the basis The most plausible argument for Stylites Transpiration does not necessarily imply of evolution of not possessing stomata is that it lost them the presence of stomata ( compare with the crassulacean acid metabolism through submergence, not that its ances­ extant Polytrichum and the Siluro­ tors never possessed them. Devonian Cooksonia), but it does indicate 1 KEELEY et al. have demonstrated clearly It is true that in its type locality, Stylites that a phototroph obtains CO2 from the that the rare pteridophyte, Stylites andicola grows above the water level, but plants air rather than the sediment. The second (Isoetaceae), exhibits a variant of photo­ have been found repeatedly that grow sub­ argument relates to the presence of inter­ synthetic carbon metabolism which has merged. The lack of an efficient method cellular air spaces which seem to be corre­ obvious affinities with crassulacean acid of dispersal suggests that the plants have lated with the presence, or occurrence at metabolism (CAM). They also drew atten­ been growing in the same area for a very some stage in the plant's evolutionary his­ tion to the absence of stomata in Stylites Jong time. The substantial elevation of the tory, of epidermal pores (Marchantiales) and concluded that these findings taken high At1des in relatively recent geological or stomata (sporophytes of vascular together indicate that this unusual organ­ times could explain both their geo­ plants). These arguments suggest that Sty­ ism does not conform to the hypothesis graphical isolation and the loss of their lites was derived from transpiring ances­ that CAM may have evolved from sto­ aquatic habitats. tors that possessed stomata and derived 2 The fact that both Stylites undergo CO2 from the atmosphere rather than the matal guard cell metabolism • 3 However, as other members of the CAM similar to the aquatic species of sediment. However, Raven noted that Isoetaceae possess stomata, it may be that Isoetes is therefore not necessarily surpris­ "the sequence of acquisition of H2O­ Stylites evolved from ancestors that did ing. Stylites should not be thought of as conducting and gas-exchange/ regulation possess stomata and although it does not evidence that CAM did not evolve from mechanisms revealed by the fossil record itself possess these structures, it may stomata! guard cell metabolism. It is more is generally in accord with teleological nevertheless use pathways derived from probable that its ability to convert to CAM expectation, in that the H2O-conducting guard cell metabolism. as it invaded an aquatic environment system in the absence of the gas­ Thus the discovery of a CAM-like saved it from extinction. exchange/ regulation mechanism would photosynthetic mechanism in S. andicola be of more selective advantage than the is not necessarily inconsistent with the B. A. THOMAS reverse state of affairs". Stylites may rep­ evolution of CAM and associated path­ Department of Life Sciences, resent such a survivor. ways from stomata! guard cell metabolism. Goldsmiths' College, Thomas' conjecture about the evolution University of London, of Stylites is not consistent with our London SE14 6NW, UK observations. Although it is possible that W. COCKBURN Stylites 'lost' stomata, the argument pro­ Department of Botany, 1. Keeley, J. E., Osmond, C. B. & Raven, J. A. Nature 310, posed by Thomas is not plausible; he University of Leicester, 694-695 (1984). 2. Moore, P. D. Nature 310,633 (1984). points out that deep-water Isoetes (for Leicester LEI 7RH, UK 3. Pigg, K. B.& Rothwell,G. W. Bot. Oaz. 144, 132-142 (1983). example, I. lacustris) have totally lost 4. Masarati, D. L. & Thomas, B. A. Pl,yta, 155-162 (1982). stomata and that the lack of stomata in I. Keeley, J. E., Osmond, C. B. & Raven, J. A. Nature 310, Stylites may derive from a previous aqua­ 694-695 (1984). 2. Cockburn, W. Pl. Cell Envir. 4, 417-418 (1981). tic ancestry. Although aquatic Isoetes species from deep water are astomatous, KEELEY ET AL. REPLY-We agree with all such species ~o far tested (including I. Cockburn that, although the presence of macrospora, considered to be conspecific 4 1 with I. lacustris ) will produce stomata on THE suggestion by Keeley et al. , suppor­ CAM in astomatous Stylites does not sup­ 2 leaves initiated under 'artificial' aerial ted by Moore , that Stylites may represent port his theory on the stomata! origin of conditions (J.E.K., unpublished data). an important link between aquatic plants CAM, it does not necessarily falsify the The important feature of the astomatous and primitive land plants has little founda­ theory either. habit of Stylites is the reliance on sediment tion in fact. To go further and propose We also agree with Thomas that Stylites carbon. Data on I. triquetra from the high­ that these plants could be relics of a mar­ most reasonably should be included with altitude paramo of the northern Andes ginally successful compromise from which Isoetes, a taxon dating back only to the (J.E.K., unpublished) and descriptions of present-day terrestrial photosynthetic sys­ Triassic. Lycophytes, however, extend the recently discovered I. hopei Croft from tems were selected runs contrary to our back to the Devonian. Although it has the topical alpine of New Guinea suggest views on plant evolution. Stylites, whose been proposed that Isoetes represents a that this syndrome, although rare, may be separation from Isoetes is questionable, is reduction from a tree-like lycopod, it is widespread globally in the Isoetaceae. regarded generally as a descendant of a clear that herbaceous lycophytes have long line of lycophytes and should not be been present since the Carboniferous and J. E. KEELEY thought of as primitive plants. Specifically, form the longest unbroken time record of 1 Department of Biology, they most probably evolved from woody­ any group of vascular plants • We do not Occidental College, stemmed plants with cormose bases, such believe that Stylites (Isocetes) andicola Los Angeles, as the Carboniferous swamp plant was the first vascular land plant, but it California 90041, USA Chaloneria. These early lycophytes cer­ may represent a model system for how vascular plants invaded seasonally dry ter­ tainly possessed stomata3. Isoetes-like C. B. OSMOND plants existed in the Triassic and probably restrial environments. Desert Research Institute, spread along the coastal plains before The view of the morphological evo­ University of Nevada, moving inland. These are terrestrial lution of Stylites which Thomas presents Reno, Nevada 89506, USA agrees with the known fossil record of the species of Isoetes, shallow-water-living 2 species (of which some have emergent lsoetales (see Thomas and Brack-Hanes ). J. A. RAVEN sporophylls and others are totally sub­ It is also consistent with two arguments Department of Biological Sciences, 3 merged) and species living on the bottom proposed by Raven as to the physiologi­ University of Dundee, of deep-water Jakes. This migration into cal significance of morphological Dundee DDT 4HN, UK water resulted in the loss of stomata! func­ attributes in vascular plants, the first of tion in shallow-water-living species (for which relates to the presence of xylem I. Banks, H. P. BioScience 25, 730-737 (1975). tissue in Stylites. The probable earliest 2. Thomas, B. A. & Brack-Hanes, S. D. Taxon 33, 247-255 example, Isoetes malinverneana) and the (1984). total Joss of stomata in the deeper living function for xylem was that of water con­ 3. Raven, J. A. Bot. J. Linn. Soc. 88, l05-126 ( 1984). 4 4. Kott, L. & Britton, D. M. Can. J. Bot. 61, 3140-3-163 (1983). species (for example, Isoetes lacustris) • duction rather than mechanical support. © 1985 Nature Publishing Group