The Elements of Botany Part 24

[Ill.u.s.tration: Fig. 516. Plant of Isoetes. 517. Base of a leaf and contained sporocarp filled with microspores cut across, magnified. 518.

Same divided lengthwise, equally magnified; some microspores seen at the left. 519. Section of a spore-case containing macrospores, equally magnified; at the right three macrospores more magnified.]

490. Investigation of this prothallus under the microscope resulted in the discovery of a wholly unsuspected kind of fertilization, taking place at this germinating stage of the plant. On the under side of the prothallus two kinds of organs appear (Fig. 510). One may be likened to an open and depressed ovule, with a single cell at bottom answering to nucleus; the other, to an anther; but instead of pollen, it discharges corkscrew-shaped microscopic filaments, which bear some cilia of extreme tenuity, by the rapid vibration of which the filaments move freely over a wet surface. These filaments travel over the surface of the prothallus, and even to other prothalli (for there are natural hybrid Ferns), reach and enter the ovule-like cavities, and fertilize the cell. This thereupon sets up a growth, forms a vegetable bud, and so develops the new plant.

491. An essentially similar process of fertilization has been discovered in the preceding and the following families of Pteridophytes; but it is mostly subterranean and very difficult to observe.

492. =Club-Mosses or Lycopodiums.= Some of the common kinds, called Ground Pine, are familiar, being largely used for Christmas wreaths and other decoration. They are low evergreens, some creeping, all with considerable wood in their stems: this thickly beset with small leaves.

In the axils of some of these leaves, or more commonly, in the axils of peculiar leaves changed into bracts (as in Fig. 511, 512) spore-cases appear, as roundish or kidney-shaped bodies, of firm texture, opening round the top into two valves, and discharging a great quant.i.ty of a very fine yellow powder, the spores.

493. The Selaginellas have been separated from Lycopodium, which they much resemble, because they produce two kinds of spores, in separate spore-cases. One kind (MICROSPORES) is just that of Lycopodium; the other consists of only four large spores (MACROSPORES), in a spore-case which usually breaks in pieces at maturity (Fig. 513-515).

494. =The Quillworts, Isoetes= (Fig. 516-519), are very unlike Club Mosses in aspect, but have been a.s.sociated with them. They look more like Rushes, and live in water, or partly out of it. A very short stem, like a corm, bears a cl.u.s.ter of roots underneath; above it is covered by the broad bases of a cl.u.s.ter of awl-shaped or thread-shaped leaves. The spore-cases are immersed in the bases of the leaves. The outer leaf-bases contain numerous macrospores; the inner are filled with innumerable microspores.

[Ill.u.s.tration: Fig. 520. Plant of Marsilia quadrifoliata, reduced in size; at the right a pair of sporocarps of about natural size.]

495. =The Pillworts= (_Marsilia_ and _Pilularia_) are low aquatics, which bear globular or pill-shaped fruit (SPOROCARPS) on the lower part of their leaf-stalks or on their slender creeping stems. The leaves of the commoner species of Marsilia might be taken for four-leaved Clover.

(See Fig. 520.) The sporocarps are usually raised on a short stalk.

Within they are divided lengthwise by a part.i.tion, and then crosswise by several part.i.tions. These part.i.tions bear numerous delicate sacs or spore-cases of two kinds, intermixed. The larger ones contain each a large spore, or macrospore; the smaller contain numerous microspores, immersed in mucilage. At maturity the fruit bursts or splits open at top, and the two kinds of spores are discharged. The large ones in germination produce a small prothallus; upon which the contents of the microspores act in the same way as in Ferns, and with a similar result.

496. =Azolla= is a little floating plant, looking like a small Liverwort or Moss. Its branches are covered with minute and scale-shaped leaves.

On the under side of the branches are found egg-shaped thin-walled sporocarps of two kinds. The small ones open across and discharge microspores; the larger burst irregularly, and bring to view globose spore-cases, attached to the bottom of the sporocarp by a slender stalk.

These delicate spore-cases burst and set free about four macrospores, which are fertilized at germination, in the manner of the Pillworts and Quillworts. (See Fig. 521-526.)

[Ill.u.s.tration: Fig. 521. Small plant of Azolla Caroliniana. 522. Portion magnified, showing the two kinds of sporocarp; the small ones contain microspores. 523 represents one more magnified. 524. The larger sporocarp more magnified. 525. Same more magnified and burst open, showing stalked spore-cases. 526. Two of the latter highly magnified; one of them bursting shows four contained macrospores; between the two, three of these spores highly magnified.]

497. =Cellular Cryptogams= (483) are so called because composed, even in their higher forms, of cellular tissue only, without proper wood-cells or vessels. Many of the lower kinds are mere plates, or ribbons, or simple rows of cells, or even single cells. But their highest orders follow the plan of Ferns and phanerogamous plants in having stem and leaves for their upward growth, and commonly roots, or at least rootlets, to attach them to the soil, or to trunks, or to other bodies on which they grow. Plants of this grade are chiefly Mosses. So as a whole they take the name of

498. =Bryophyta, Bryophytes= in English form, Bryum being the Greek name of a Moss. These plants are of two princ.i.p.al kinds: true Mosses (_Musci_, which is their Latin name in the plural); and Hepatic Mosses, or Liverworts (_Hepaticae_).

[Ill.u.s.tration: Fig. 527. Single plant of Physcomitrium pyriforme, magnified. 528. Top of a leaf, cut across; it consists of a single layer of cells.]

499. =Mosses or Musci.= The pale Peat-mosses (species of Sphagnum, the princ.i.p.al component of sphagnous bogs) and the strong-growing Hair-cap Moss (Polytrichum) are among the larger and commoner representatives of this numerous family; while Fountain Moss (Fontinalis) in running water sometimes attains the length of a yard or more. On the other hand, some are barely individually distinguishable to the naked eye. Fig. 527 represents a common little Moss, enlarged to about twelve times its natural size; and by its side is part of a leaf, much magnified, showing that it is composed of cellular tissue (parenchyma-cells) only. The leaves of Mosses are always simple, distinct, and sessile on the stem.

The fructification is an urn-shaped spore-case, in this as in most cases raised on a slender stalk. The spore-case loosely bears on its summit a thin and pointed cap, like a candle-extinguisher, called a _Calyptra_.

Detaching this, it is found that the spore-case is like a pyxis (376), that is, the top at maturity comes off as a lid (_Operculum_); and that the interior is filled with a green powder, the spores, which are discharged through the open mouth. In most Mosses there is a fringe of one or two rows of teeth or membrane around this mouth or orifice, the _Peristome_. When moist the peristome closes hygrometrically over the orifice more or less; when drier the teeth or processes commonly bend outward or recurve; and then the spores more readily escape. In Hair-cap Moss a membrane is stretched quite across the mouth, like a drum-head, retaining the spores until this wears away. See Figures 527-541 for details.

500. Fertilization in Mosses is by the a.n.a.logues of stamens and pistils, which are hidden in the axils of leaves, or in the cl.u.s.ter of leaves at the end of the stem. The a.n.a.logue of the anther (_Antheridium_) is a cellular sac, which in bursting discharges innumerable delicate cells floating in a mucilaginous liquid; each of these bursts and sets free a vibratile self-moving thread. These threads, one or more, reach the orifice of the pistil-shaped body, the _Pistillidium_, and act upon a particular cell at its base within. This cell in its growth develops into the spore-case and its stalk (when there is any), carrying on its summit the wall of the pistillidium, which becomes the calyptra.

[Ill.u.s.tration: Fig. 529. Mnium cuspidatum, smaller than nature. 530. Its calyptra, detached, enlarged. 531. Its spore-case, with top of stalk, magnified, the lid (532) being detached, the outer peristome appears.

533. Part of a cellular ring (_annulus_) which was under the lid, outside of the peristome, more magnified. 534. Some of the outer and of the inner peristome (consisting of jointed teeth) much magnified. 535.

Antheridia and a pistillidium (the so-called flower) at end of a stem of same plant, the leaves torn away (?, antheridia, ?, pistillidium), magnified. 536. A bursting antheridium, and some of the accompanying jointed threads, highly magnified. 537. Summit of an open spore-case of a Moss, which has a peristome of 16 pairs of teeth. 538. The double peristome of a Hypnum. 539-541. Spore-case, detached calyptra, and top of more enlarged spore-case and detached lid, of Physcomitrium pyriforme (Fig. 527): orifice shows that there is no peristome.]

501. =Liverworts or Hepatic Mosses= (_Hepaticae_) in some kinds resemble true Mosses, having distinct stem and leaves, although their leaves occasionally run together; while in others there is no distinction of stem and leaf, but the whole plant is a leaf-like body, which produces rootlets on the lower face and its fructification on the upper. Those of the moss-like kind (sometimes called Scale-Mosses) have their tender spore-cases splitting into four valves; and with their spores are intermixed some slender spiral and very hygrometric threads (called _Elaters_) which are thought to aid in the dispersion of the spores.

(Fig. 542-544.)

[Ill.u.s.tration: Fig. 542. Fructification of a Jungermannia, magnified; its cellular spore-stalk, surrounded at base by some of the leaves, at summit the 4-valved spore-case opening, discharging spores and elaters.

543. Two elaters and some spores from the same, highly magnified.]

[Ill.u.s.tration: Fig. 544. One of the frondose Liverworts, Steetzia, otherwise like a Jungermannia; the spore-case not yet protruded from its sheath.]

502. Marchantia, the commonest and largest of the true Liverworts, forms large green plates or fronds on damp and shady ground, and sends up from some part of the upper face a stout stalk, ending in a several-lobed umbrella-shaped body, under the lobes of which hang several thin-walled spore-cases, which burst open and discharge spores and elaters. Riccia natans (Fig. 545) consists of wedge-shaped or heart-shaped fronds, which float free in pools of still water. The under face bears copious rootlets; in the substance of the upper face are the spore-cases, their pointed tips merely projecting: there they burst open, and discharge their spores. These are comparatively few and large, and are in fours; so they are very like the macrospores of Pillworts or Quillworts.

503. =Thallophyta, or Thallophytes= in English form. This is the name for the lower cla.s.s of Cellular Cryptogams,--plants in which there is no marked distinction into root, stem, and leaves. Roots in any proper sense they never have, as organs for absorbing, although some of the larger Seaweeds (such as the Sea Colander, Fig. 553) have them as holdfasts. Instead of axis and foliage, there is a stratum of frond, in such plants commonly called a THALLUS (by a strained use of a Greek and Latin word which means a green shoot or bough), which may have any kind of form, leaf-like, stem-like, branchy, extended to a flat plate, or gathered into a sphere, or drawn out into threads, or reduced to a single row of cells, or even reduced to single cells. Indeed, Thallophytes are so multifarious, so numerous in kinds, so protean in their stages and transformations, so recondite in their fructification, and many so microscopic in size, either of the plant itself or its essential organs, that they have to be elaborately described in separate books and made subjects of special study.

[Ill.u.s.tration: Fig. 545, 546. Two plants of Riccia natans, about natural size. 547. Magnified section of a part of the frond, showing two immersed spore-cases, and one emptied s.p.a.ce. 548. Magnified section of a spore-case with some spores. 549. Magnified spore-case torn out, and spores; one figure of the spores united; the other of the four separated.]

504. Nevertheless, it may be well to try to give some general idea of what Algae and Lichens and Fungi are. Linnaeus had them all under the orders of Algae and Fungi. Afterwards the Lichens were separated; but of late it has been made most probable that a Lichen consists of an Alga and a Fungus conjoined. At least it must be so in some of the ambiguous forms. Botanists are in the way of bringing out new cla.s.sifications of the Thallophytes, as they come to understand their structure and relations better. Here, it need only be said that

505. Lichens live in the air, that is, on the ground, or on rocks, trunks, walls, and the like, and grow when moistened by rains. They a.s.similate air, water, and some earthy matter, just as do ordinary plants. Algae, or Seaweeds, live in water, and live the same kind of life as do ordinary plants. Fungi, whatever medium they inhabit, live as animals do, upon organic matter,--upon what other plants have a.s.similated, or upon the products of their decay. True as these general distinctions are, it is no less true that these orders run together in their lowest forms; and that Algae and Fungi may be traced down into forms so low and simple that no clear line can be drawn between them; and even into forms of which it is uncertain whether they should be called plants or animals. It is as well to say that they are not high enough in rank to be distinctively either the one or the other. On the other hand there is a peculiar group of plants, which in simplicity of composition resemble the simpler Algae, while in fructification and in the arrangements of their simple cells into stem and branches they seem to be of a higher order, viz.:--

[Ill.u.s.tration: Fig. 550. Branch of a Chara, about natural size. 551. A fruiting portion, magnified, showing the structure; a sporocarp, and an antheridium. 552. Outlines of a portion of the stem in section, showing the central cell and the outer or cortical cells.]

506. =Characeae.= These are aquatic herbs, of considerable size, abounding in ponds. The simpler kinds (Nitella) have the stem formed of a single row of tubular cells, and at the nodes, or junction of the cells, a whorl of similar branches. Chara (Fig. 550-552) is the same, except that the cells which make up the stem and the princ.i.p.al branches are strengthened by a coating of many smaller tubular cells, applied to the surface of the main or central cell. The fructification consists of a globular sporocarp of considerable size, which is spirally enwrapped by tubular cells twisted around it: by the side of this is a smaller and globular antheridium. The latter breaks up into eight shield-shaped pieces, with an internal stalk, and bearing long and ribbon shaped filaments, which consist of a row of delicate cells, each of which discharges a free-moving microscopic thread (the a.n.a.logue of the pollen or pollen-tube), nearly in the manner of Ferns and Mosses. One of these threads reaches and fertilizes a cell at the apex of the nucleus or solid body of the sporocarp. This subsequently germinates and forms a new individual.

507. =Algae or Seaweeds.= The proper Seaweeds may be studied by the aid of Professor Farlow's "Marine Algae of New England;" the fresh-water species, by Prof. H. C. Woods's "Fresh-water Algae of North America," a larger and less accessible volume. A few common forms are here very briefly mentioned and ill.u.s.trated, to give an idea of the family. But they are of almost endless diversity.

[Ill.u.s.tration: Fig. 553. Agarum Turneri, Sea Colander (so called from the perforations with which the frond, as it grows, becomes riddled); very much reduced in size.]

[Ill.u.s.tration: Fig. 554. Upper end of a Rockweed, Fucus vesiculosus, reduced half or more, _b_, the fructification.]

508. The common Rockweed (Fucus vesiculosus, Fig. 554, abounding between high and low water mark on the coast), the rarer Sea Colander (Agarum Turneri, Fig. 553), and Laminaria, of which the larger forms are called Devil's Ap.r.o.ns, are good representatives of the olive green or brownish Seaweeds. They are attached either by a disk-like base or by root-like holdfasts to the rocks or stones on which they grow.

[Ill.u.s.tration: Fig. 555. Magnified section through a fertile conceptacle of Rockweed, showing the large spores in the midst of threads of cells.

556. Similar section of a sterile conceptacle, containing slender antheridia. From Farlow's "Marine Algae of New England."]

509. The hollow and inflated places in the Fucus vesiculosus or Rockweed (Fig. 554) are air-bladders for buoyancy. The fructification forms in the substance of the tips of the frond: the rough dots mark the places where the conceptacles open. The spores and the fertilizing cells are in different plants. Sections of the two kinds of conceptacles are given in Fig. 555 and 556. The contents of the conceptacles are discharged through a small orifice which in each figure is at the margin of the page. The large spores are formed eight together in a mother-cell. The minute motile filaments of the antheridia fertilize the large spores after injection into the water: and then the latter promptly acquire a cell-wall and germinate.

510. The Florideae or Rose-red series of marine Algae (which, however, are sometimes green or brownish) are the most attractive to amateurs. The delicate Porphyra or Laver is in some countries eaten as a delicacy, and the cartilaginous Chondrus crispus has been largely used for jelly.

Besides their conceptacles, which contain true spores (Fig. 560), they mostly have a fructification in _Tetraspores_, that is, of spores originating in fours (Fig. 559).

[Ill.u.s.tration: Fig. 557. Small plant of Chondrus crispus, or Carrageen Moss, reduced in size, in fruit; the spots represent the fructification, consisting of numerous tetraspores in bunches in the substance of the plant. 558. Section through the thickness of one of the lobes, magnified, pa.s.sing through two of the imbedded fruit-cl.u.s.ters. 559. Two of its tetraspores (spores in fours), highly magnified.]

[Ill.u.s.tration: Fig. 560. Section through a conceptacle of Delesseria Leprieurei, much magnified, showing the spores, which are single specialized cells, two or three in a row.]

[Ill.u.s.tration: Fig. 561. A piece of the rose-red Delesseria Leprieurei, double natural size. 562. A piece cut out and much magnified, showing that it is composed of a layer of cells. 563. A few of the cells more highly magnified: the cells are gelatinous and thick-walled.]

511. The Gra.s.s-green Algae sometimes form broad membranous fronds, such as those of the common Ulva of the sea-sh.o.r.e, but most of them form mere threads, either simple or branched. To this division belong almost all the Fresh-water Algae, such as those which const.i.tute the silky threads or green slime of running streams or standing pools, and which were all called Confervas before their immense diversity was known. Some are formed of a single row of cells, developed each from the end of another.

Others branch, the top of one cell producing more than one new one (Fig.

564). Others, of a kind which is very common in fresh water, simple threads made of a line of cells, have the chlorophyll and protoplasm of each cell arranged in spiral lines or bands. They form spores in a peculiar way, which gives to this family the designation of conjugating Algae.

[Ill.u.s.tration: Fig. 564. The growing end of a branching Conferva (Cladophora glomerata), much magnified; showing how, by a kind of budding growth, a new cell is formed by a cross part.i.tion separating the newer tip from the older part below; also, how the branches arise.]

[Ill.u.s.tration: Fig. 565. Two magnified individuals of a Spirogyra, forming spores by conjugation; a completed spore at base: above, successive stages of the conjugation are represented.]

512. At a certain time two parallel threads approach each other more closely; contiguous parts of a cell of each thread bulge or grow out, and unite when they meet; the cell-wall part.i.tions between them are absorbed so as to open a free communication; the spiral band of green matter in both cells breaks up; the whole of that of one cell pa.s.ses over into the other; and of the united contents a large green spore is formed. Soon the old cells decay, and the spore set free is ready to germinate. Fig. 565 represents several stages of the conjugating process, which, however, would never be found all together like this in one pair of threads.

[Ill.u.s.tration: Fig. 566. Closterium acutum, a common Desmid, moderately magnified. It is a single firm-walled cell, filled with green protoplasmic matter.]

[Ill.u.s.tration: Fig. 567. More magnified view of three stages of the conjugation of a pair of the same.]