Cells and tissues in the vegetative sporophytes of early land plants

Abstract

SUMMARY Remarkable preservation in coalified and pennineralized fossils from Upper Silurian and Lower Devonian sediments deposited some 420 to 390 million years ago provides insight into the major anatomical innovations associated with the early stages in the colonization of the land by higher plants. Using uniformitarian principles, such information, combined with gross morphology, can then be used to reconstruct the pioneers as growing, metabolizing and reproducing organisms, as well as allowing assessment of affinity, although apart from the lycophytes, they have no close relationship with extant groups. In considering vascular tissues, diversity is exemplified by descriptions of the metaxylem in protosteles of Zosterophyllopsida and Drepanophycopsida (putative lycophytes), of Psilophyton (Trimerophytopsida), of the Rhyniaceae (including Rhynia gwynnevaughanii, certain Taeniocrada spp., Sennicaulis) and of Cooksonia pertoni, a Lower Devonian representative of the organization found in the oldest pteridophyte‐like land plants. Aglaophyton major is included as a plant with bryophyte‐like vascular tissues in a branching sporophyte with cuticle, stomata and intercellular space system typical of the homoiohydric tracheophyte. In all water‐conducting cells, interpretation of the primary and secondary wall results from comparisons involving the anatomy and chemistry of extant examples and an understanding of taphonomic processes. Phloem is only rarely preserved and usually identified from its position around the xylem. In contrast, dermal features are better known, because of penetration of the resilient cuticle between epidermal cells. They appear conservative. Thus stomata with two guard cells look remarkably similar to extant forms in surface view, and by consideration of their relationship with surrounding epidermal cells and of extent of cuticularization, seem to have operated in a similar way to those in certain mosses and ferns. Cuticular ornamentation (papillae, striations) and epidermal outgrowths (unicellular and multicellular) are described and their adaptive significance conjectured. Ground tissue systems are best preserved in Rhynie Chert permineralizations where zonation of the cortex is tentatively related to photosynthetic and structural roles. The latter function is also attributed to thick‐walled, outer cortical cells recorded in many zosterophylls and in Psilophyton, although the chemical nature of the walls themselves remains equivocal. Problems of identification of roots in plants possessing axial organization with exarch xylem, and in leafy plants with smooth lateral branching systems are briefly addressed. Finally axis apices in Rhynia givynne‐vaughanii and sections showing stages in the maturation of Asteroxylon stems are described from the Rhynie Chert.

The Formation and Function of Plant Cuticles

“…Consequently, the capacity to synthesize, deposit, and maintain a hydrophobic surface layer, or cuticle, over the surfaces of aerial organs was arguably one of the most important innovations in the history of plant evolution. This idea is borne out by both fossil evidence (Edwards, 1993) and the ubiquity of cuticles among all extant embryophytes, from bryophytes (Budke et al, 2012) to angiosperms.…”

Nondestructive,in situ, cellular-scale mapping of elemental abundances including organic carbon in permineralized fossils

“…P ermineralized fossils preserve biological remains in threedimensional cellular detail, enabling paleontologists to document the morphology and anatomy of ancient organisms that range from cyanobacteria (1) and the embryos of early animals (2, 3) to both sporophytes and gametophytes of early land plants (4)(5)(6). The anatomical resolution provided by permineralized fossils presents opportunities for fine-scale chemical analysis of preserved organic matter, paving the way toward cell-and tissue-level studies of paleobiochemistry (7)(8)(9)(10).…”

The role of Mid-Palaeozoic mesofossils in the detection of early bryophytes

“…In a re-evaluation of the central strand of Aglaophyton, D. S. Edwards (1986) concluded that (i) the innermost cells were comparable with polytrichaceous hydroids; (ii) they were surrounded by stereids with a presumed structural role; and (iii) the outermost tissue had similarities with moss leptoids. However, it should be emphasized that such an arrangement has no extact counterpart in extant bryophytes (Edwards 1993). The strand of smooth, thin-walled cells described here is closer in wall dimensions to those surrounding the central zone of putative hydroids.…”

“…Structures less likely to be preserved are rhizoids. These are preserved by silica in the Rhynie Chert and are unicellular (see review in Edwards 1993) in both tracheophytes (e.g. Rhynia, Trichopherophyton) and Aglaophyton, which is one of the few Rhynie Chert taxon reputed to have some bryophyte characters.…”