Sexual reproduction
- Related Topics:
- Equisetopsida
- Psilotopsida
- Marattiidae
- sun fern
- Polypodiidae
The sex organs of ferns are of two types. The sperm-producing organ, the antheridium, consists of a jacket of sterile cells with sperm-producing cells inside. Antheridia may be sunken (as in the families Ophioglossaceae and Marattiaceae) or protruding. They vary in size from those with hundreds of sperm to those with only 12 or so. The egg-producing organ, the archegonium, contains one gamete (sex cell), which is always located in the lower, more or less dilated portion of the archegonium, the venter. The upper part of the archegonium, the neck, consists of four rows of cells containing central neck cells. The uppermost of the neck cells are the neck canal cells; the lowest cell is the ventral canal cell, which is situated just above the egg.
Fertilization is attained by the ejection of sperm from antheridia. The sperm swim through free water toward simple organic acids released at the opening of the archegonium, the neck of which spreads apart at the apex, permitting the neck cells to be extruded and the sperm to swim in and penetrate the egg. The sperm are made up almost entirely of nuclear material, but their surface is provided with spiral bands of cilia—hairlike organs that effect locomotion. When the egg is fertilized, the base of the neck closes, and the embryo develops within the expanding venter.
Embryo
Within the archegonial venter the zygote undergoes characteristic cell divisions to form the embryo, which remains encapsulated in the gametophyte until it breaks out and becomes an independent plant. The pattern of development in most ferns is a distinctive one, and indeed only in the Botrychium subgenus Sceptridium and in all species of the family Marattiaceae thus far studied are found conditions of embryonic development resembling those of seed plants. Here the first division of the zygote is transverse. The inner cell grows inward, producing the stem and first leaf, and the outer cell divides to form a foot, a mass of tissue that exists as part of the embryo and disappears when its function, presumably absorption, is completed. The root appears later within the stem and grows outward. In all other known ferns the zygote divides neatly into four quadrants, the first division approximately parallel to the long axis of the archegonium and the following division at right angles. This results in initial cells that give rise to four organs: the outer forward cell (i.e., toward the growing apex of the gametophyte and the neck of the archegonium) becomes the first leaf, the inner forward cell the stem apex, the outer back cell the first root, and the inner back cell the foot. Thus, the majority of ferns tend to have a precise arrangement of their organs and the divisions that produce them in the embryo.
The young sporophytes of ferns remain attached to the gametophytes for varying lengths of time, absorbing nutrients from the gametophyte through the foot. Once the sporophyte has developed independent existence and the root has penetrated the soil, the gametophyte soon shrivels.
Stem
Fern stems vary from the tall, narrow trunks of certain tree ferns that reach 25 metres (80 feet) tall down to clumped or creeping rootstocks, or rhizomes. Rhizomes are the most common stem form. The majority of them grow horizontally upon or just beneath the surface of the soil. Some stems are so narrow as to be threadlike, as in many tropical epiphytic ferns. A few ferns in different parts of the world have evolved radically specialized stems containing chambers in which ants take up residence; the role of the ants in the lives of these ferns is unknown, but it may be for protection against other insects. Vinelike ferns are common, but shrubby ferns are extremely rare.
Stem growth is initiated by one to several large apical cells. These are usually well protected by various types of hairs or scales and by the overarching embryonic leaves. Leaves and leaf bases play a major role in the protection of fern stems, and many stems are said to have a leaf armour. Such stems are densely covered with old sclerified leaf bases, which increase the apparent size of the stem many times. The old leaf bases may serve as protection or as food-storage organs. In most species the stems are indeterminate in growth and thus can theoretically continue to grow indefinitely. Annuals—short-lived species that complete development, shed spores, and die in a single growing season—are exceptional; only a few examples are known.
Surface structure
Whether covered with leaf armour or not, the surface of the fern stem is protected by an epidermis, or “skin,” a single layer of epidermal cells, which are more or less flat cells with thick outer walls. Most fern stems also are covered with a protective indument, consisting of hairs, known as trichomes, or scales; these are so distinctive that they are valuable in identification and classification. The indument includes such diverse types of epidermal emergences as simple glands (unbranched one- to several-celled trichomes with a headlike cluster of secretory terminal cells), simple (unbranched) nonglandular trichomes, dendroid trichomes (branching filaments), and scales (flat cell plates) of many patterns. Scales (also known as paleae) are each defined as a cell plate two or more cell rows wide, at least at the base, whereas hairs each generally consist of a single row of cells. Transitional states are also known.
Cortex
The cortex is the region outside the vascular cylinder but below the surface of the stem. It is composed mostly of storage parenchyma cells (a relatively generalized cell type). Rooting animals, such as pigs, occasionally dig up fern rhizomes for the starchy materials contained in them. There is a strong tendency for the outermost cortical cells to become darkly pigmented and thick-walled.
Vascular tissues
The steles—cylinders of vascular tissues in the centres of fern stems—exhibit somewhat diverse patterns. Most common ferns possess a “dictyostele,” consisting of vascular strands interconnected in such a manner that, in any given cross section of stem, several distinct bundles can be observed. These are separated by regions filled with parenchyma cells known as leaf gaps. There are, however, numerous “siphonostelic” ferns, in which the gaps do not overlap and a given section shows only one gap, and some “protostelic” ferns, in which no gaps at all are formed. Complex stelar patterns are known in some species, as in the common bracken fern (Pteridium), which has a polycyclic dictyostele, in which one stele occurs within another stele. Large strands of fibrelike cells running between the two steles form mechanically specialized hard tissue, or sclerenchyma. As herbaceous plants, ferns do not form true woody tissues.
Root
Fern roots are generally thin and wiry, although some are fleshy and either slender (in the Ophioglossaceae) or as much as 13 mm (0.5 inch) in diameter (e.g., Acrostichum and Marattia). The relation of the roots to the stems is a valuable identification tool. For example, in certain tree ferns (e.g., Cyathea and Cibotium) and in the royal ferns (Osmunda), the entire stem surface is covered by masses of roots. If large enough, the dead tangles of tree fern roots can be cut with a saw into various shapes suitable for attaching epiphytic greenhouse plants, and pieces of such root masses have proved to be useful in horticulture for cultivating orchids and bromeliads. Because of the massive destruction of tree ferns for this purpose, the importation of tree fern logs into many countries is now prohibited. Certain tropical ferns have elaborately hairy roots whose surfaces are covered with locks of silky golden or brown root hairs.
Leaf
Shapes
The leaf (also known as frond) of ferns is the part of the plant most readily visible to observation. The leaf plan in practically all ferns is pinnate—that is, featherlike with a central axis and smaller side branches—and this is considered to be the primitive condition because of its widespread occurrence. From this basic type a broad diversity of forms evolved. Some ferns have palmate leaves (with veins or leaflets radiating from one point), and some, such as the staghorn ferns, have secondarily evolved falsely dichotomous leaves. In some genera (e.g., Lygodium and Salpichlaena) the main leaf axis (rachis) twines about on shrubs and small trees, sometimes reaching 20 metres (65 feet) in length.
Whether a given leaf is divided into segments (compound) or is undivided (simple) is of considerable value in identification of similar fern species. The difference between divided and undivided leaves is not a profound one, however, and closely related species commonly differ from one another in this respect.
The extent of division in fern leaves, or fronds, ranges from those in which the leaf margins are merely so deeply lobed as to have narrow-based segments to those having obviously stalked leaflets, or pinnae. The pinnae themselves may also be lobed or truly divided with stalked segments; and the resulting segments, the pinnules, may also be lobed or divided. Depending on the degree of cutting, fronds are described as simple, once divided, twice divided, thrice divided, and so on. Some ferns are known in which the fronds are five times compound, making them exceedingly delicate, with segments so small as to be almost hairlike.
Venation
Generally, the patterns of the leaf veins, or vascular bundles (which can be seen readily by holding the specimen up to a strong light), are pinnate, and the veins are free; that is, they all diverge and never coalesce, either along their sides or at the ends. Nevertheless, there are numerous fern groups in which netted, or reticulate, venation is found. These have vein patterns like those of other ferns for the most part, except that various systems of networks and areolae (areas enclosed within loops of veins) have developed between the major, pinnately arranged veins. There are many reticulate patterns known. One of the more striking is that in which each loop or areola contains one or more free included veinlets, as seen in various members of the family Polypodiaceae. Another is the herringbone pattern, believed to result from an evolutionary concrescence (growing together) of pinnae, as shown by certain tree ferns (Cyathea), lady ferns (Athyrium), and marsh ferns (Thelypteris).
