UNIT II EVOLUTION AND DIVERSITY 88 CHAPTER 4 EVOLUTION AND DIVERSITY OF VASCULAR PLANTS I OF PLANTS 89 apex of the expanded base, most sporophyllous. Sporangia microsporophylls and megasporophylls, respectively), grouped are heterosporous, and are located on the adaxial side of leaf together in terminal strobili, the sporophylls in four rows, not (sporophyll) bases; megasporangia occur on outer leaves of much differentiated from vegetative leaves. Gametophytes a flush of growth, the megaspores large (50—300 per sporan are endosporic. gium), trilete, spore sculpturing used in species identification; The Selaginellaceae are mostly distributed in tropical and microsporangia occur on inner leaves (or in alternating cycle warm regions, worldwide. Economic importance includes with megasporangia), the microspores small, monolete, very cultivated ornamentals and local medicinal plants. See Jermy numerous (up to I million per sporangium). Both sporangia (1990b) for general infonnation and Korall and Kenrick (2002, are marginally covered by a membrane, the “velum”, and are 2004) for phylogenetic analyses of the family. internally traversed by sterile strands (“trabeculae”); sporan The Selaginellaceae are distinctive in being erect to pros gia lack a precise dehiscence mechanism and open by tissue trate herbs, with dichotomously branched stems, sometimes degradation. Gametophytes are endosporic. Plants have CAM forming planar branch systems, the leaves microphyllous, photosynthesis. Air chambers occur in roots and leaves. spiral, either homomorphic or dimorphic and four-rowed The Isoetaceae have a worldwide distribution. Economic (with two upper rows of leaves smaller than the two lower importance is limited to some cultivated ornamentals. See rows), sporangia heterosporous, microsporangia and me Jermy (1990a) for general information and Rydin and Wik gasporangia borne in axils of ligulate sporophylls of terminal strom (2002) and Hoot et al. (2006) for phylogenetic and strobili; gametophytes endosporic. biogeographic studies of the family. The Isoetaceae are distinctive in being cormose to rhi EUPHYLLOPHITA—EUPHYLLOPHYTE S zo,natous plants with a basal rosette of microphyllous, ligu The sister group of the lycophytes are the euphyllophytes, late leaves, the leaves basally sheathing, apically linear to including all the other extant vascular plants (Figure 4.1). acicular, heterosporous, bearing adaxial megasporangia or Several major apomorphies that unite the euphyllophytes are microsporangia within sheathing leaf base. mentioned here. First, in contrast to the lycophytes, the roots [Note that Isoetes and Isoetaceae can be spelled Isoëtes and of euphyllophytes are monopodial, meaning that they do not Isoëtaceae, respectively, the umlaut indicating that the “e” is dichtomously branch at the apical meristem. Lateral roots a separate vowel and should be pronounced, not part of the arise endogenously from either the endodermis (in monilo diphthong “oe.” See Botanical Names, in Chapter 16.] phytes) or the pericycle (in spermatophytes, Chapter 5). Second, the roots of euphyllophytes have an exarch protoxy Selaginellaceae—Spike-Moss family (Latin Selago, a moss- 1cm, in which the protoxylem is positioned outer to the like plant of the Scrophulariaceae + ella, diminutive). 1 genus metaxylem (Figure 4.20A,B); lycophyte roots have an endarch (Selaginella)Ica. 700 species. (Figure 4.19) protoxylem. Third, the ancestral sporangia in euphyllophytes The Selaginellaceae consist of perennial herbs, rarely tree- were terminal in position with longitudinal deshiscence like, some species xeric-adapted “resurrection plants” (e.g., S. (although these features have undergone considerable modi lepidophylla). The roots are adventitious and dichotomously fication in some groups). Fourth, extant euphyllophytes have branching (dichopodial), in some taxa arising from branch a molecular apomorphy, a 30-kilobase inversion located in junctions and growing downward (formerly interpreted as leaf the large single-copy region of chloroplast DNA (Figure less stems, termed “rhizophores”). The stems are generally di 4.20C; see Figure 14.4 of Chapter 14). Fifth, the leaves of chotomously branching, with erect, cespitose, prostrate/repand, euphyllophytes, termed euphylls, are distinctive. (Note that or climbing habit; the stems may be pseudomonopodial or euphyll is essentially synonymous with megaphyll, a more sympodial, forming a very flattened, “fern-like” branch system traditional term.) Euphylls, like lycophylls, are generally in some species, some with aerial tubers; the stem vasculature is dorsiventral organs, functioning as the primary organ of pho a protostele (exarch or mesarch). The leaves are simple, sessile, tosynthesis. Euphylls are different in being associated with a spiral, with a single midrib (microphyllous), adaxially ligul ate, leaf gap, a region of nonvascular, parenchyma tissue inter - blades generally small, either homomorphic (“isophyllous”) rupting the vasculature of the stem, and in (usually) having or, in some prostrate taxa, dimorphic (“anisophyllous”) and in more than one vein per leaf (Figure 4.20F). Euphylls gener FIGURE 4.19 LYCOPODIOPHYTA—ISOETOPSIDA. Selaginellaceae. A. Selaginella bigelovii, with isomorphic leaves. B—K. four rows, leaves of two upper (dorsal) rows smaller, those of ally have a highly branched system of veins, between which Selagineila spp. B. Shoot with dimorphic leaves. C. Close-up of vegetative shoot, showing 2 rows of large and 2 rows of small leaves. the other two lower (ventral or lateral) rows larger. Sporangia is the mesophyll, the chloroplast-containing tissue. (Note D. Close-up of ligule, adaxial side of leaf base. E. Cone (strobilus), an axis bearing microsporophylls and megasporophylls. F. Close-up of megasporangium (sporophylls removed). G. Adaxial view of microsporophyll and megasporophyll with axillary are heterosporous; microsporangia (bearing numerous, small, that in a few euphyllous taxa, the veins have become second microsporangium and microsporangium and megasporangium, respectively. H. Strobilus longitudinal-section, showing sporophylls, megasporangia, and microspo trilete microspores) and megasporangia (bearing usually four arily reduced to a single mid-vein, an evolutionary reversal.) rangia. I. Close-up of microsporangium, containing numerous microspores. J. Close-up of megasporangium, containing 4 megaspores. [numerous], large, trilete. gen. ornamented megaspores) oc In addition, euphylls, in contrast to lycophylls, grow by means K. Dispersed microspores and megaspores, the latter showing trilete mark. Note great size difference. cur on short stalks in the axils of ligulate sporophylls (termed of either marginal or apical meristems. UNIT II EVOLUTION AND DIVERSITY 88 CHAPTER 4 EVOLUTION AND DIVERSITY OF VASCULAR PLANTS I OF PLANTS 89 apex of the expanded base, most sporophyllous. Sporangia microsporophylls and megasporophylls, respectively), grouped are heterosporous, and are located on the adaxial side of leaf together in terminal strobili, the sporophylls in four rows, not (sporophyll) bases; megasporangia occur on outer leaves of much differentiated from vegetative leaves. Gametophytes a flush of growth, the megaspores large (50—300 per sporan are endosporic. gium), trilete, spore sculpturing used in species identification; The Selaginellaceae are mostly distributed in tropical and microsporangia occur on inner leaves (or in alternating cycle warm regions, worldwide. Economic importance includes with megasporangia), the microspores small, monolete, very cultivated ornamentals and local medicinal plants. See Jermy numerous (up to I million per sporangium). Both sporangia (1990b) for general infonnation and Korall and Kenrick (2002, are marginally covered by a membrane, the “velum”, and are 2004) for phylogenetic analyses of the family. internally traversed by sterile strands (“trabeculae”); sporan The Selaginellaceae are distinctive in being erect to pros gia lack a precise dehiscence mechanism and open by tissue trate herbs, with dichotomously branched stems, sometimes degradation. Gametophytes are endosporic. Plants have CAM forming planar branch systems, the leaves microphyllous, photosynthesis. Air chambers occur in roots and leaves. spiral, either homomorphic or dimorphic and four-rowed The Isoetaceae have a worldwide distribution. Economic (with two upper rows of leaves smaller than the two lower importance is limited to some cultivated ornamentals. See rows), sporangia heterosporous, microsporangia and me Jermy (1990a) for general information and Rydin and Wik gasporangia borne in axils of ligulate sporophylls of terminal strom (2002) and Hoot et al. (2006) for phylogenetic and strobili; gametophytes endosporic. biogeographic studies of the family. The Isoetaceae are distinctive in being cormose to rhi EUPHYLLOPHITA—EUPHYLLOPHYTE S zo,natous plants with a basal rosette of microphyllous, ligu The sister group of the lycophytes are the euphyllophytes, late leaves, the leaves basally sheathing, apically linear to including all the other extant vascular plants (Figure 4.1). acicular, heterosporous, bearing adaxial megasporangia or Several major apomorphies that unite the euphyllophytes are microsporangia within sheathing leaf base. mentioned here. First, in contrast to the lycophytes, the roots [Note that Isoetes and Isoetaceae can be spelled Isoëtes and of euphyllophytes are monopodial, meaning that they do not Isoëtaceae, respectively, the umlaut indicating that the “e” is dichtomously