Pteridology is the study of ferns—plants classified in the Division Pterophyta (or Filicophyta). Ferns do not have seeds the way trees and flowering plants do. Rather, they have spores the way mosses do. The haploid spores grow small haploid organisms, which then undergo fertilization and grow the diploid fern plant directly out of the haploid gametophyte, similar to the sporophyte stalk growing out of the moss. The larger part, what we think of as the fern, is the sporophyte. The gametophyte is a small green prothallus that the sporophyte grows out of. Ferns are still tied to an aquatic environment, in that once a spore grows into a prothallus, there must be moisture enough for the egg in the prothallus to be fertilized by swimming, flagellated fern sperm.
Having a large sporophyte allows ferns to produce many more spores than a moss could- recall that each sporophyte on a moss only carried one sporangia. Producing many more propagules increased fern presence and dominance. Besides having a larger sporophyte generation, ferns have many important adaptations that increase their capabilities above the mosses. Ferns have roots, which, unlike moss rhizoids, not only anchor, but take up nutrients. Ferns are vascular plants, with lignified vascular tissues. These allow active water transport. That water transport along with the strength of the ligified cells allow ferns to be much larger than their moss ancestors. At one point, ferns and fern trees were the most advanced plant life, and grew even larger than ferns today do, with great size and variety of ferns. There were no flowering plants in the early cretaceous- the first forests of the dinosaurs were composed of fern trees.
The Equisetophyta is a very old division—the plants of this division were among the first plants to grow on land. Equisetophyta are sometimes referred to as fern allies, as they have the same life cycles as ferns, and have similar developments that allow them to grow taller than the bryophytes.
The Division Pinophyta in the Kingdom Plantae comprises those species of plants that were formerly classified as the “modern” gymnosperms of the Class Coniferales—that is the conifers. Unlike many of the gymnosperm groups covered in the previous chapter,pinophytes are today still broadly represented on the landscape, forming extensive forests in both the Northern and Southern hemispheres. This does not mean this is a recent group in the paleontological record. Pinophytes are found as fossils as far back as the Upper Carboniferous (Paleozoic Era). Thus, it is a very ancient group, but one still having significant ecological importance on the planet. Pinophytes are mostly evergreen trees (some are shrubs) and many have great commercial value for their wood.
Reread Gymnosperm (Links need not be pursued at this time)
Read Conifers (Links need not be pursued at this time)
These terms, gymnosperm and conifer no longer have standing in modern taxonomic treatments of plants. However, both terms are still widely used, so you should have a grasp of what they mean, and how they fit into the taxonomic terms that have replaced them.
Read Division Pinophyta (The following links are included:)
Evolution of the Gymnosperms
Gymnosperms are very different from the earliest vascular plants. Gymnosperms have very reduced gametophyte generations- female ovules and male pollen. Many gymnosperms, like the Pinophytes, have secondary growth from a vascular cambium. While tree ferns are tall and have a trunk resembling a tree, they are only very superficially similar. They do not have woody growth the way trees with secondary growth from a vascular cambium do. A single mutation of a fern or a horsetail could not produce a functioning seed plant. So gymnosperms must be descended from some progymnosperm ancestor that evolved these adaptations from the ferns, but was not fit enough to remain on earth. One possible example of a progymnosperm is the spore bearing woody tree Archaeopteris, which was at one point probably prevalent on earth.
The Division Magnoliophyta in the Kingdom Plantae comprises those species of plants that were formerly classified as angiosperms and are known widely as the flowering plants. You have already studied flowers (Chapter 4), so now understand that the Division Magnoliophyta comprises all those species of plants that have flowers. For perspective: all of the plants we have read about in Section II of the Botany Study Guide up to this point do not have flowers, but certainly do have reproductive structures. We also know that flowers are the reproductive structures of the plants that bear them, and that reproductive structures are not limited to flowering plants. Thus, “flowers” are structures that distinguish plants in the Division Magnoliophyta from plants in the other divisions of the Kingdom Plantae. Observe a flower, and you know you are examining an angiosperm. However, not all angiosperms have obvious, showy flowers. You will need to consider that the structure of a flower is quite variable across all the many species of angiosperms (about a quarter million have been identified) and a few flowering plants actually seldom flower. However, angiosperm botanists put great stock in the structure of flowers as a way of classifying plants—more than any other part of a plant, the flower provides the basis for placement of a species in subtaxa (classes, orders, and families) of the Division Magnoliophyta.
The Division Magnoliophyta is split into two large classes: the Magnoliopsida and theLiliopsida.
Flower evolution
Conifers are pollinated by wind, meaning they must produce a large amount of pollen grains for only a few to arrive at the female megaspore, resulting in fertilization. An advancement that allowed for higher rates of fertilization per energy expended in making pollen would surely result in that new plant type being prolific and even dominant. However, a single mutation would never result in a flower adequately adapted to spread pollen using animals. Flowers are the result of a special kind of evolution called co-evolution.
One possible explanation of how flowers evolved from conifers follows. Peggyhopper (talk) 02:26, 28 November 2010 (UTC)Peggy Hopper Suppose one wind pollinated plant began to have its’ pollen eaten by an animal, for example a beetle. Then suppose the beetle spreads the pollen from the male cones to the female cones while looking for pollen because it can’t tell the difference between the male and female cones; it searches both, spreading pollen from the male to the female in doing so. This plant has an advantage over all wind pollinated plants, because of a higher rate of fertilization. Any mutation that leads to pollen being spread by an animal soon becomes prevalent within a species, because the plant pollinated by an animal is more efficient. The plant and the animal rely on each other, one for food, the other for reproduction. Any mutation in the animal that helps pollinate the plant will become prolific through natural selection, just as mutations that favor feeding the insect or making the insect come to the flower will be dominant. This back and forth evolution results in such seemingly improbable structures as flowers that look like female moths that are pollinated by amorous male moths, or flowers and bird beaks that complement the feeding of the bird and the pollination of the flowers.
Class Magnoliopsida (dicots)
Members of the Class Magnoliopsida are defined partly on the basis of the seed or seedling having two cotyledons, most obvious at germination. But the differences between dicots and monocots are many, and we will be able to recognize most flowering plants that we encounter as belonging to one or the other class without having to dissect the seed or observe the seedling.
The Class Liliopsida constitutes the monocotyledonous angiosperms and includes some of the largest plant families such as the orchids with some 20,000 species and the grasses with perhaps 15,000 species. In the previous chapter we learned how to separate the two major flowering plant groups: the dicotyledons and the monocotyledons, and covered a number of dicot families. In this chapter, we shall do the same by considering representative monocot families.
The liliopsids are considered to form a monophyletic group evolved from an early dicot. The oldest fossils presumed to be monocot remains date from the Early Cretaceous (Herendeen & Crane, 1995). Features that are generally common to monocots include vascular bundles that are irregularly distributed (in cross-section of the stem), leaves with parallel venation, and flower parts in multiples of three. Although true secondary growth is absent, most growth habits are found in the group including floating and submerged aquatics, lianas, trees, epiphytes, and forbs of all sizes (Hahn, 2005).
The second largest (to Orchidaceae) and one of the most successful of families of monocotyledons is the grass family, classified as the Family Poaceae (or Gramineae) and comprising nearly 10,000 species distributed more widely than any other angiosperm family. This family is also the most important economically, providing species that are the world’s staple food supply. The grasses have reduced floral structures compared with most angiosperms for the reason that grasses are almost exclusively pollinated by wind. Therefore, these plants have had no cause to evolve floral structures that are attractive to insect (or other animal) pollinators. The grasses also have a fairly specific body plan that is immediately recognizable and very successful for colonizing seasonally dry landscapes, yet modifiable to suit a wide range of ecological conditions. Bor (1960) stated:
The amazing diversity of shapes, sizes, texture and so on in the various parts that make up the vegetative and the reproductive shoot is alike the admiration and exasperation of the taxonomist. It can be said… that in no other family of monocotyledons, or dicotyledons for that matter, has a relatively simple structure been so altered in the process of evolution as to produce the amazing wealth of forms known to us. It is as if Evolution, in an excess of exuberance, had added this, subtracted that, enlarged this, suppressed that, in fact tried out every possible combination and permutation of characters to produce a family as complicated and as difficult [taxonomically] as, if not more difficult than, any other.
Grasses have fibrous roots and three kinds of stems: culms, rhizomes, and stolons. The culm is the main aerial shoot to which leaves and flower head are attached. The culm is a rounded or slightly flattened stem with one or more solid joints known asnodes. The leaves are attached at the nodes and if the stem is not simple but branched, branches arise only at nodes. Roots may also develop from a node where the node comes into contact with the ground (as in decumbent and prostrate stems). The portion of the stem between the nodes is called the internode, and is usually hollow in temperate zone grasses and solid in tropical grasses (Rotar, 1968). All or a portion of an internode may be surrounded by the basal part of the leaf known as the sheath.
rhizome is a modified stem that grows underground. Rhizomes are jointed (thus distinguishable from roots) with bladeless leaves (scales) arising from the joints. Rhizomes enable the grass plant to spread horizontally as new culms develop vertically from the joints. Thus, grasses with extensive rhizome development will form a turf rather than distinct tufts or bunches.
stolon is a stem that creeps across the surface of the ground, and is really a basal branch of the culm that will develop roots and shoots from some or all of its nodes. Like a rhizome, a stolon results in a spreading or turf forming grass plant.
Poa trivialis showing membranous ligules pressed against culm
Grasses display two types of leaves: (1) green leaves consisting of a sheath and blade, and (2) reduced leaves consisting of only a sheath. With but a few exceptions, the green leaves arise at nodes alternately up the culm. Leaves that are concentrated near the base where the internodes are very short are termed basal leaves; leaves arising at nodes along an elongated culm are cauline leaves. These vegetative leaves typically surround the culm as a sheath, then diverge outward (at the “collar”) as a long narrowblade with longitudinal parallel venation. If the veins are conspicuous, the leaf is striate; if the veins are raised, the leaf is ribbed.
The sheath of the leaf surrounds and protects the shoot. In some species, the sheath extends beyond the next node, so that consecutive leaf sheaths overlap, hiding the nodes. The longitudinal edges of the sheath may overlap, completely surrounding the culm, or the sheath may be tubular (the margins connate). The upper end of the sheath, known as the sheath mouth is the collar on the lower (outer) surface that may be produced into short appendages called auricles. On the inner, upper surface of the leaf, between the sheath and the blade, is an outgrowth called a ligule. This may be a flap of membranous tissue or simply a fringe of hairs, an inconspicuous rim, or even absent all together, marked only by dark tissue.
Although there is variation in leaf blade shape, most grasses have linear-shaped leaves that are many times longer than wide, with margins that are parallel then taper to a point at the apex. Grasses that grow in shady places may have lanceolate or even ovate leaf blades.
Flowers of grasses are borne in an inflorescence or flower head which terminates the culm and other branches of the stem. Smaller units of the inflorescence are calledspikelets and these are arranged on one or more branches in a wide variety of different ways to which the standard terminology for inflorescences can be applied, but using the spikelet instead of the individual flower. Thus:
In a panicle, spikelets are carried on branches of the axis.
The panicle can be either open or dense and crowded, with the branches so short that they are concealed by the spikelets, forming a false spike.
In a spike, the spikelets are carried, unstalked, directly on an unbranched axis, called a rachis.
In a raceme some or all of the spikelets are attached to the rachis by stalks calledpedicels.