Mature Sporangia and Annulus Structure

sions in a moss, the FlG - 29 - Mature sporangia, closed (A) and chromosome number is open reduced. All the cells of the jacket are thin-walled except those of one row. Each of the cells composing this row has thick walls on all sides but the outer one. This row of cells, extending from the base of the capsule up one side, over the top, and partly down 370 GENERAL BOTANY the other side, is the annulus. When the spores are mature, the cells of the jacket are dead and dry. The cell walls of the annulus are sensitive to changes in moisture. As a result of such changes the annulus straightens, breaking open the capsule, and then snaps forward. In this lat- ter movement most of the spores are thrown out. 283. Gametophyte. The spores of the bracken ripen and are shed in late summer. Each spore is approx- imately tetrahedral in shape, and its wall has two layers: the inner one thin, the outer hard, brown, and ir- regularly thickened. When a spore germi- nates, the thick outer layer of the wall FIG. 291. A, B 1 early stages in the development of a prothallium from a fern spore. C, half- grown prothallium. breaks, and the pro- toplast, surrounded ^y ^e jnner layer of / the wall, . lorms a , short, green outgrowth from which a colorless projection, the first rhizoid, grows. As a result of growth and of cell divisions in one plane, the green outgrowth becomes, usually, a row of three or four cells (Fig. 291, A). By subsequent growth and by cell divisions in two planes, the young plant, except for the few cells nearest the old spore wall, is transformed into a flat, green plate one cell in thickness. If this small prothallium is not crowded during its further growth, it develops typically into a heart-shaped plant with a shallow A notch at its anterior end (Fig. 291, C). mature prothallium (Figs. 292, 293) is one cell in thickness, except that in a region back of the apical notch a cushion several cells thick is formed. From various cells of the under surface of the plant, and partic- ularly in the older portion (that farthest from the notch), slender, colorless rhizoids grow out which anchor the plant and absorb FILICINEAE (FERNS) 371 water and other materials from the soil. Prothallia may reach maturity in a few months, but they remain so small that they are A rchegonia Apica I Notch Antheridia fihizoid -/ I i FIG. 292. Mature fern prothallium viewed from below. rarely observed in nature unless sought for. Fully grown prothallia are often not more than a quarter inch in diameter. The prothallium is the sexual generation or gametophyte of a fern, and like the gametophyte of a moss it produces gametes. In Posterior nd Anterior End Archegonia FIG. 293. Vertical lengthwise section of a fern prothallium. some species of mosses, gametes of both kinds are borne by the same gametophyte; in other species, antherozoids and eggs are borne on separate plants. The bracken is like the mosses of the former type, in that antherozoids and eggs may be produced by the same gametophyte. If prothallia are small and poorly nour- 372 GENERAL BOTANY ished, they often form only antheridia and antherozoids, but such prothallia may develop archegonia and eggs if they are placed under better conditions for food-making. Antheridia may occur on almost any part of the plant but are most numerous on the under surface, particularly on the posterior portion of the pro- CeUaqf Prothallium Immature Antherozoids ^-^ Antheridial' Jacket . ^ B FIG. 294. A, antheridium of a fern. B, antherozoid. thallium, where rhizoids are abundant. Archegonia are borne also on the under surface, but only on the cushion of cells back of the apical notch. An antheridium (Fig. 294, A) is dome-shaped and much smaller than an antheridium of a moss. The few cells of its outer layer FIG. 295. A, nearly mature archegonium of a fern. B, archegonium at the time of the entrance of antherozoids. constitute a jacket. After a series of cell divisions, each interior cell develops into an antherozoid (Fig. 294, B) which is larger than the antherozoid of a moss and has the form of a short spiral. Borne FILICINEAE (FERNS) 373 on its slender anterior portion are many flagella by means of which the antherozoid swims rapidly. An archegonium (Fig. 295) has essentially the same structure as an archegonium of a moss, but is smaller and composed of fewer cells. Its venter is imbedded in the cushion of the prothallium. Its neck is short and usually curves backward from the notch toward the older portion of the prothallium. At maturity, the cells of the canal row disintegrate and the cap cells of the neck break apart, leaving a passage-way to the egg. Although sex organs 'Remains of Archegonium *PrimcaM Root Prin Leaf A FIG. 296. t early stage in the development of a fern embryo within the venter of an archegonium. B, an embryo, still partly within the archegonium, differentiated into foot, primary root, primary leaf, and stem. of both kinds are produced on the same prothallium, most of the antheridia usually develop and discharge their antherozoids before the archegonia on the same plant have matured. Hence the union of gametes from different plants, rather than from the same plant, is probably the rule in the bracken. Such union is made possible by the fact that the prothallia grow in groups in moist places. 284. Development of an Embryo (Fig. 296). Though many antherozoids may reach the mouth of an archegonium and enter the neck, only one unites with the egg. The zygote, like that of a moss, germinates within the venter of the archegonium. After a few divisions, forming a small mass of cells, the young sporo- phyte (embryo) becomes four-lobed. By further division and growth, one lobe develops into a foot, a small organ imbedded in the prothallial cushion; from this cushion the foot absorbs food for the embryo. Another lobe develops into a primary root, which 374 GENERAL BOTANY pushes downward through the surrounding tissues and grows into A the soil. third lobe gives rise to a primary leaf which, growing outward and forward beneath the prothallium, turns upward at the notch and develops a green blade much simpler in form than the blades of the leaves to be produced later. The stem develops slowly from the fourth lobe. Until Blade of Primary Leaf Petiole of Primary Leaf the time of the production of the primary root and primary leaf, the embryo (young sporophyte) has been parasitic upon the gametophyte (Fig. 297). With the full development of the primary leaf and primary root, however, the sporophyte becomes an independent plant, and somewhat later the gametophyte dies. The stem grows slowly into the soil, producing secondary Primary Root Rhizoids leaves and secondary roots. After the formation of several secondary leaves and second- FIG. 297. Young sporophyte still attached d parasitic upon ' the gameto" Jhyte ary roots, the primary leaf and the Primary root die Thus the mature sporophyte has been derived from only one lobe of the embryo, namely, that which developed into the stem. 285. Life Cycle (Fig. 298). The life cycle of the bracken, like A that of a moss, includes "two distinct phases. spore gives rise to a minute green plant, the gametophyte, which forms sex organs bearing gametes. The union of gametes (antherozoid and egg) forms a zygote, which on germination produces an embryo para- sitic upon the gametophyte. By further growth the embryo de- velops into a large, independent plant, the sporophyte, consisting of stem, roots, and leaves. On some of the leaves are borne sporan- gia which contain spores, completing the cycle. While the histories of moss and fern are alike in general outline, there are important differences. The gametophyte of a moss is relatively large and may live for a number of years, whereas its sporophyte is relatively small and short-lived. In a fern the game- FILICINEAE (FERNS) 375 tophyte is very small and comparatively short-lived, while the sporophyte is large and may live for many years. The conspicuous moss plant is the gametophyte; the conspicuous fern plant is the sporophyte. OTHER FERNS 286. Leaves. Although all ferns are alike in the general characteristics that indicate their relatively close relationship, different FIG. 298. Life cycle of a fern. species vary markedly in form and structure. The differences are evident chiefly in the sporophytic generation. The gametophytes of most ferns are essentially like the gametophyte of the bracken. One conspicuous feature in which ferns differ from one another is in the form of the leaf blade. The " walking 7 fern' represents a A type whose blades are not lobed or divided. leaf blade of this fern has the shape of a greatly elongated triangle whose slender tip grows until it bends over and comes into contact with the soil. When the tip touches the soil it develops a small bud that gives A rise to roots and a stem, and so produces a new plant. repetition 376 GENERAL BOTANY of this process by the successively formed new plants explains the name given this fern. Some ferns, such as the common polypody, have simple but very deeply lobed leaf blades. Others, including the royal fern, the lady fern, and the male fern, produce pinnately divided leaves, each primary leaflet being also pinnately lobed or divided. The petiole of the maidenhair fern is forked at the summit, each of the two divisions so formed bearing on one side several spreading, pinnately divided leaflets. 287. Sporangia. The sporangia of most ferns, unlike those of the bracken, are produced in rounded or linear groups (sori) on the "V,/' ^ ri '^r^^'^'ts'A ,,'''.' j < t 't ,t r ';' . .Lett, tne interrupted tern, itignt, trie grape tern. under surfaces of leaf blades. Each sorus is borne upon an elevated cushion of tissue, and in some ferns is covered by a variously shaped outgrowth of the leaf. In the bracken, as has been seen, many leaves bear sporangia that is, are sporophylls; those leaves which are sterile otherwise resemble the sporophylls. In the interrupted fern (Fig. 299) the production of sporangia is confined to several pairs of leaflets near the middle of the blade. These leaflets are small and brown, and on their margins are borne numerous sporangia; the other (sterile) FILICINEAE (FERNS) 377 leaflets are larger and green. After the spores are shed in the early summer the spore-bearing leaflets wither. The sporangia of the royal fern are borne on a few leaflets at the apex of the leaf blade. A leaf of the grape fern (Fig. 299) consists of two distinct parts: one is a flat, much-divided blade which performs most of the photosynthetic work; the other part of the leaf has as its function only the production of spores. In a few species, including the cinnamon fern, the sensitive fern (Fig. 300), and the ostrich fern, there are two different kinds of leaves. Those of one sort, the sterile leaves, are broad and green and are photosynthetic organs; those of the ABC D FIG. 300. Fertile (A-C) and sterile (D) leaves of the sensitive fern. other sort, the sporophylls, are brown at maturity and function only in spore-production. 288. Roots (Fig. 301). The roots of all ferns are essentially similar in the structure and arrangement of their tissues. In the mature region of a root a relatively thick cortex encloses a small central stele. On its outer side the cortex is bounded by an epider- mis one cell in thickness. The cortical cells are for the most part thin-walled and often contain numerous starch grains. The cells of the endodermis are distinguished by thickenings on their radial walls. Next within the endodermis is the pericycle, consisting of 378 GENERAL BOTANY one or two layers of thin-walled cells. At various points just within the pericycle are strands of xylem separated from one another (in a young portion of the mature region) by thin-walled cells. While the first-maturing portions of these xylem strands, consisting of small cells, are adjacent to the pericycle, the latermaturing portions extend the strands inward toward the center of the stele so that in most cases a central solid mass of xylem is eventually formed. Just within the pericycle and alternating with Pericycle Phloem Endodermis First-Matured Xyletn Later-Matured Xylem FIG. 301. Cross section of a portion of the stele and cortex in a root of Angiopteris. the first-maturing strands of xylern are small strands of phloem. Such a stele, in which the xylem forms a more or less solid central column with the phloem on its outer side, is a protostele. The arrangement of xylem just described is an exarch arrangement. Exarch xylem is characterized by the fact that the firstmatured xylem elements are at the outer margin of the stele adjacent to the pericycle, the later-maturing xylem elements lying A inward from those first matured. protostele with such a xylem- arrangement is an exarch protostele. All roots, both in pteridophytes and in seed plants, have exarch steles; in many cases these are protosteles; but in others, the center of the root is occupied by pith, 289. Stems. The stems of ferns, unlike their roots, vary greatly in external form and in internal structure. Many ferns growing in temperate regions, like the bracken, have underground stems. In the tropics, however, there is greater diversity. Here some FILICINEAE (FERNS) 379 species grow perched on the limbs and branches of trees. The stems of other species are prostrate on the ground or clamber upon other plants. Still other species have stems beneath the soil. In the tropics also are found tree ferns (Fig. 285). An exarch protostele seems to represent the most primitive type from which all other types of stele have been derived. As has been seen, the primitive exarch protostele is still characteris- tic of roots; in the stems of most pteridophytes and seed plants, however, other stelar types derived from the exarch protoetele are found. The stems of only a few ferns have protosteles; one of these is Gleichenia flabellata, which grows chiefly in the tropics and subtropics. The stem of Gleichenia has a thick-walled epidermis, beneath which is a cortex some of whose cells have relatively thick A walls. single-layered endodermis encloses the pericycle, which may be several cells in thickness. Within the pericycle is a thin cylinder of phloem; and entirely enclosed by the phloem is the A xylem which fills the center of the stele. few parenchymatous cells are scattered through the xylem. This protostele differs from that of a root in the course of development of its xylem. The first xylem elements to mature are, as in a root, groups of small cells developing at various points within the pericycle. However, the later-maturing xylem not only develops to the center of tiie stele but also surrounds the first-matured strands. This arrange- ment, in which the later-maturing xylem surrounds the first- matured strands, is a mesarch arrangement. Whereas the root of a fern contains an exarch protostele, the stem of Gleichenia has a mesarch protostele. The maidenhair fern (Adiantum pedatwri) illustrates a still more advanced type of stem structure. Its stele encloses a central pith. In the stele, midway between pith and cortex, is a continuous cylinder of xylem. On both inner and outer faces of the xylem are, successively, phloem, pericycle, and endodermis. This type of cylindrical stele with a central pith is a siphonostele. The siphono- stele of Adiantum has both internal and external phloem; in most siphonosteles, however, phloem is present only on the outer side of the xylem. The xylem of Adiantum, like that of Gleichenia, is mesarch. Adiantum, therefore, has a mesarch siphonostele. Just as the mesarch condition has been derived from the exarch, so an endarch condition, in which the first-matured xylem elements are on the 380 GENERAL BOTANY FIG. 302. Diagrams of various types of fern steles; the, first-matured xylem in all cases is in black, the later-matured xylem diagonally shaded, the phloem stippled; pith and cortex are unshaded. A, exarch protostele. B, mesarch protostele. (7, D, mesarch siphonosteles with (C) phloem on E both sides, and (D) on the outer side only, of the xylem. endarch t F siphonostele. t dictyostele. FILICINEAE (FERNS) 381 inner face of the xylem zone, has been derived from the mesarch. The endarch condition is characteristic of the stems of nearly all living seed plants. As a young leaf of Adiantum grows and develops, a vascular strand is formed, extending outward from the stele through the cortex of the stem and into the petiole. Beyond the junction of leaf trace and stele there is in the stele of the stem a long, slender area in which parenchymatous cells are formed instead of xylem and phloem. This elongated interruption of the stele just ahead of the junction of each leaf trace,, in which only parenchymatous cells are formed, is a leaf gap. In many ferns leaf gaps are long and nu- merous, giving to the stelar cylinder the form of a network of bundles with elongated meshes. In cross section such a network has the appearance of separate bundles arranged in a circle about a central pith. This type of stele is a dictyostele. Such a stele occurs in the stem of Polypodium. The arrangement of bundles in the stem of the bracken represents a modified type of dictyostele. 290. Bryophytes and Ferns. About 150 genera and more than 6,000 species of ferns are known. They constitute a class standing conspicuously higher than the bryophytes in the sense that they have advanced further from a primitive condition. Like the bryophytes the ferns have a distinct alternation of generations". The sporophyte of a bryophyte is small, relatively simple, and always attached to and largely dependent upon the gametophyte. The sporophyte of a fern, on the other hand, is a relatively large, complex plant differentiated into stem, leaves, and roots, and therefore independent of the gametophyte. The fern gametophyte, however, does not show a corresponding development. Although an independent green plant, it is always very small and simple in structure. In spite of the radical change in relative size and complexity whereby the sporophyte has become the large, conspicuous generation, the gametophyte still retains the function of producing gametes and the sporophyte continues to produce spores. CHAPTER XXVII SOME OTHER PTERIDOPHYTES EQTJISETUM 291. Nature. The few living species of Equisetum (" horsetails ") are related to a group of plants which, during one period of the earth's history, formed a conspicuous feature of its vegetation. Some of these ancient plants developed into goodsized trees, but the present- day species of Equisetum are mostly small. In tropical South America, the stems of one species grow to a height of more than 30 feet. Its stems are, however, very slender and lean upon the shrubs and trees among which they grow. Equisetum is almost world-wide in its distribu- tion and thrives in a vari- ety of habitats. Certain species grow in ponds and in swamps ; others in mead- ows and in damp, shaded Rhizome' A places; and still others in relatively dry and exposed FIG. 303. Sporophyte of Equisetum. ,4,ster- situations Slich BS sandy ile aerial branch. ,B, fertile aerial branch. embankments. C, sporophyll. Z>, E, spores with spiral 292. Sporophyte. Equi- bands uncoiled (D) and coiled (E). setum arvense (Fig. 303) is common in habitats of the type last mentioned. The sporophyte of this species is composed of a horizontal, branching underground stem and of aerial branches, some sterile and some fertile, which grow upward from the nodes of the stem. The underground stem 382 SOME OTHER PTER1DOPHYTES 385 when dry and coiling about the spore when moist. The bands of several spores may become entangled; the spores, therefore, are shed in small clusters and m&y germinate to form groups of game- tophytes. 294. Gametophyte and Embryo. The germination of a spore results, as in a fern, in the formation of a small green prothallium A (Fig. 305) which bears sex organs. prothallium of Equisetum differs from that of a fern in its form and in the location of the sex organs. When mature, it is usually a disk-shaped cushion several cells in thickness, from whose upper surface arise irreg- ularly lolbed, flattened branches each one cell thick. Rhizoids grow from the lower surface of the cushion. Antheridia are borne usually near the apices of the vertical branches; archegonia, on the upper surface of the cushion at the bases of the branches. An antherozoid resembles one of a fern in having many flagella. An embryo in its early stages of development is in most re- spects similar to that of a fern. Lobes of a young embryo develop respectively into a foot, a primary stem bearing two to four pri- mary leaves, and a primary root. The primary stem remains very small. At its base a bud arises that grows into a larger branch. In like manner at the base of this first branch a second branch arises, and the process may be repeated. Eventually one of the later- formed branches grows downward, penetrates the soil, and de- velops into the characteristic stem from which sterile and fertile aerial branches subsequently arise. CLUB MOSSES: LYCOPODIUM 295. Sporophyte. The club mosses also are related to an old group of plants once very abundant. Some of the ancient club mosses were tree-like. The present-day members of the group are all small. The plants familiarly known as "club mosses," "ground pines/' and "Christmas greens" are members of the genus Lyco~ podium (Fig. 306). The various species of this genus occur in tropical as well as in temperate regions. Some tropical and subtropical species grow on trunks and branches of trees. Those of temperate regions grow on the ground. The form of the sporophyte differs somewhat according to the species. Often it has a branching stem which creeps over the surface of the ground or lives within the soil, producing slender roots and sending up aerial branches. The aerial stems and branches are usually well covered with small, 386 GENERAL BOTANY narrowly triangular, sessile leaves. The leaves are relatively ( simple in structure, being only a few cells in thickness. Stomata are present; the internal cells of a leaf are all similar excepting those of the phloem and xylern, which compose an unbranched vein or midrib extend- ing from the base of the leaf part-way toward the apex. The stem has an epidermis, a thick cortex, and a stele. The xylem and phloem of the stele are in plates whose arrangement varies with the species as well as with the direction of growth of the stem. 296. Spore-forma- tion. On the inner side, and near the base, of each of certain leaves is a small sporangium (Fig. 307, C). Such leaves are sporophylh. In some species of Lycopodium, the sporophylls are not read- ily distinguishable, either by their appear- ance or by their posi- FIG. 306. Sporophyte of Lycopodium. tion, from the sterile (foliage) leaves. In other species the sporophylls are borne, more or less compactly grouped, on the terminal portions of some of the upright branches, which thus constitute strobili (Fig. 307, A, B). In such a case the sporophylls are smaller, and contain a lesser proportion of chloro- phyll, than the foliage leaves. Each sporangium has a short stalk, and a jacket several cells in thickness. Within the sporangium are developed many spore mother cells, each of which finally divides to form four spores. SOME OTHER PTERIDOPHYTES 387 297. Gametophyte and Embryo. The spores seem to lie dormant for several years after being shed. In the majority of species, a spore on germination forms a small subterranean, saprophytic gametophyte (Fig. 308), on whose upper portion are borne antheridia and archegonia. In general plan the sex organs resemble