Part II

girdle imbedded in the peripheral part of the dense cytoplasm. The chloroplast contains one, two, or several pyrenoids. Near the center of the cell is a large nucleus. Reproduction of cells takes place in Ulothrix, as in other plants, by means of cell division. The cells divide by constriction. Division increases the number of cells in the colony but not the number 210 GENERAL BOTANY of colonies. The number of colonies may be increased, however, by an accidental breaking of the filaments. Reproduction of the colony (that is, the formation of now colonies) is brought about also by the production of motile cells (swarm-spores). With the exception of a few cells at the base, any cell of a filament may divide to form 2, 4, 8, 16, or 32 small naked cells. These daughter cells, at first angular, become rounded, form flagella, and, enclosed in a vesicle, escape through a pore in the side of the parent-cell wall (Fig. 149, A). These swarm-spores are similar to the motile cells of Chlamydomonas, each being ovoid and having a prominent chloroplast and a conspicuous pigment spot. Unlike the motile cells of Chlamydomonas, however, each swarm-spore has four flagella instead of two and is without a cell wall. After swimming for some time the spore comes to rest on some solid body, withdraws its flagella, forms a wall, and pushfcs out a protuberance which is the beginning of the formation of a holdfast. The growth and transverse division of this cell, of its daughter cells, and of their offspring, the cells always remaining A in contact, give rise to a new filament. cell of a colony may also, under some conditions, become an immobile rounded spore pro- vided with a wall; or it may first divide to produce two or more such spores. A spore of this type may develop directly into a new filament, or may be transformed into a ^warm-spore which will so develop. 144. Gametic Union. Ulothrix also produces gametes. These are formed in the same manner as are swarm-spores, and are similar to the latter except that they are frequently smaller and that each has two instead of four flagella (Fig. 149, B). After swimming for a time, the gametes unite in pairs (Fig. 149, C-fJ). In this union the flagella do not disappear, so that each zygote, having four flagella, continues moving about after its formation. Eventually it comes to rest, withdraws its flagella, secretes a wall, and, after a short period of rest, divides to form several (at least four) non-motile spores, each of which, like a swarm-spore, de- velops into a new filamentous colony. 145. Ulva. Certain green algae with cells similar to those of Ulo- thrix differ from that alga in the structure of their colonies. One of the most striking of these is Ulva, the "sea lettuce" (Fig. 150). CHLOROPHYCEAE (GREEN ALGAE) 211 This plant commonly grows on rocks and wharves in brackish or salt water. The thallus, composed of Ulothrix-like cells, is an irregularly expanded sheet, often with a surface area of several square inches but only two cells in thickness. The plant is anchored at its basal end by a very irregularly shaped holdfast composed of elongated colls or rows of cells. Ulva produces four-flagellate swarm -spores and two- f1 agellatc gametes . CLADOPHORA 146. Structure and Reproduction. Some filamentous green algae differ from Ulothrix both in cell structure and in colonial organization. An example is seen in Cla- dophora (Fig. 151, A), which grows attached to objects in streams arid in shallow water along the shores of lakes. The cylindrical cells are united end to end to form a branching filament. Each cell is surrounded by^Jhicl^jvalL FIG. 150. Ulva. A, the expanded leaf- B like thallus. y portion of thallus, show- ing cell structure. Within the wall is a layer of dense cytoplasm in which are imbedded many small disk-shaped chloroplasts. Some chloro- plasts contain one pyrenoid each, others lack pyrenoids. In some species the chloroplasts appear to be united into a continuous net- work. Cladophora differs from the algae previously described in that each cell contains many nuclei. These lie imbedded in the dense cytoplasm but farther inward than the chloroplasts. Reproduction of cells is brought about through cell division by constriction. New branches are usually formed only by cells near A the upper end of a filament. branch originates as a lateral out- growth from the upper end of a cell, and the first cross wall of the new branch is formed close to the point of origin of the outgrowth. 212 GENERAL BOTANY Reproduction of the colony results from the formation of onenucleate, four-flagellate swarm-spores. These are produced by cells near the tips of branches, the protoplast of each such cell dividing to form many swarm-spores. The spores are liberated through a small pore near the upper end of the cell, or at its apex if it is the terminal cell of a branch. After swimming for a time a swarm-spore comes to rest upon some solid object, retracts its flagella, and secretes a wall. Growth and division of this cell, of its daughter cells, and of their descendants results in a new filament identical in appearance with that which produced swarm-spores. 147. Gametic Union. Gam- etes are formed and liberated in the same manner as are swarm-spores, and are similar to the latter except that each FIG. 151. Cladophora. A, portion of a sw^m-swres^ I empt^walls from which swarm-spores have escaped, B formation and liberation of gametes. hasjteo. instead ~ of -fourJjjgtgelja (Fig. 151, B). After swimming ^ ^or a ^me > e gametes unite A in* pairs to form zygotes. zygote becomes immobile and a waj} ln some species the zygote^develops immediately into a new filament which produces swarm-spores only. 148. Life Cycle. In the species of Cladophora last referred to, but apparently not in all species, the life cycle includes two dis- tinct phases. Swarm-spores develop into plants which produce gametes. Each zygote formed by gametic union develops im- mediately into a plant exactly similar to that which formed the gametes; this plant, however, produces swarm-spores only. Since the spore-bearing plant (or generation) of Cladophora gives rise through spores to the gamete-bearing generation, and the latter generation through gametes Upd zygotes gives rise to the sporebearing generation, there is Hibernation of generations. In most CHLOROPHYCEAE (GREEN ALGAE) 213 green algae there is no comparable alternation of generations, although a few others are known to have an alternation essentially similar to that just described. SPIROGYRA 149. Structure and Reproduction. Spirogyra, one of the freefloating plants commonly known as "pond scums," is a green alga whose cells form permanent filamentous colonies. It occurs in pools and other bodies of water and frequently forms masses of considerable sizelTlt may be distinguished from most other thread- like green algae by the slippery feeling of the threads, due to a gelatinous outer layer </f the cell walL) The cells of Spirogyra (Fig. 152, A) are cylindrical and attached end to end to form an unbranched thread. This arrangement results from the fact that all cell divisions take place in the same plane, namely, at right angles to the long axis of the cylindrical cells. A A thin layer of dense cytoplasm lies just within the wall of each cell. The most conspicuous feature of the cell, and the one from Chloroplast Pyrenoid Nucleus I Central Vacuole / Nucleus Pyrenoid A Gelatinous Sheath Cell Wall A Dense Cytoplasm Gelatinous Sheath Central r Vacuole t> FIG. 152. A, cell of Spirogyra. B, cross section of a cell through the nucleus, which the name Spirogyra is derived, is the chloroplast. Each chloroplast is a trough-shaped ribbjoa.extending spiraUyJTCHBPL end to end of the cell; it is part of and contained in the dense cyto- plasm.TEach chloroplast contains several pyrenoids. Throughout the length of the chloroplast is a thick central strand connecting and surrounding the pyrenoids, the intervals between successive A pyrenoids being approximately equal. central vacuole occupies the greater portion of the space within the wall. In the center of this vacuole is the nucleus, surrojM&ed by a layer of dense cytoplasm from which numerous cyt^pasmic strands extend to the? 214 GENERAL BOTANY dense cytoplasmic layer at the periphery of the cell. Each strand usually joins the peripheral layer just beneath a pyrenoid. Reproduction of the cell takes place in Spirogyra in essentially the same manner as in other plants that is, by means of cell division. Cell division in Spirogyra, under ordinary conditions, occurs at night. It is preceded by a nuclear division similar to that already described for the cells of a root tip. The division of the cells increases the number of cells in the colony but not the number of colonies. There is usually no definite means for reproduction of the colony that is, an increase in number of colonies during the vegetative life of the plant. In most species of Spirogyra new colonies are formed only when a filament is accidentally severed. Since various aquatic animals feed upon the alga, the filaments are frequently cut, so increasing the number of plants. In certain species of Spirogyra, especially in some with small cells, the fila-. ments at times become separated into individual cells or short rows of a few cells each, which may then grow into long fila- ments. 160. Gametic Union. As a rule, the production of gametes by each species of Spirogyra occurs at a definite time of the year, commonly in spring or autumn. In preparation for this process the first step observed (in most species) is a pairing of the filaments so that the filaments of each pair lie side by side. Small domeshaped protuberances now grow toward each other from opposite cells in the two filaments (Fig. 153, A, B), each protuberance increasing in size until it becomes a short tubular outgrowth. The outgrowths from opposite cells come into contact; the wall of each is digested at the point of contact, and thus a conjugation tube is formed. When the formation of the conjugation tube begins, the protoplasts (gametes) of the conjugating cells are similar in appearance, but as the protuberances grow toward each other, one of each pair of gametes contracts from the wall and becomes rounded. This change in size is brought about by a loss of water from the protoplast. The contracted gamete soon migrates through the conjugation tube (Fig. 153, (7) toward the other, which at about this time also contracts and rounds up. The gamete which contracts first and which moves toward the other gamete is spoken of as male because of its greater activity; the passive gamete is female. Usually all the cells of a particular filament which function as gametes behave as gametes of the same sex, but at times some CHLOROPHYCEAE (GREEN ALGAE) 215 of the cells in a filament become male and others in the same fila- ment female gametes. In certain species of Spirogyra, conjugation takes place between adjacent cells of the same filament rather than between cells of separate filaments. The differentiation into male and female gametes and the formation of a zygote go on, however, in the same way as when cells of different filaments conjugate. After the male gamete has migrated into the cell cavity of the female gamete', the two unite to form a zygote (Fig. 153, D-F). Both a nuclear and a cytoplasmic union are involved. The cytoplasm of the gametes seems to become intermingled; but the FIG. 153. D F Spirogyra; stages in the union of gametes and the maturing of a zygote (diagrammatic). chloroplasts do not unite, those of each gamete remaining distinct for some time. The subsequent behavior of the chloroplasts is difficult to follow, but the available evidence indicates that the chloroplast or chloroplasts contributed by the male gamete disintegrate, so that the mature zygote contains only the chloroplast or chloroplasts derived from the female gamete. The zygote soon begins to secrete a wall which, when the nuclei have united and the paternal chloroplasts have disappeared, has become thick and re- sfctant. By this time the zygote lies at the bottom of the pool or other body of water, still enclosed by the old wall of the female, gamete. The zygote eventually becomes free, since both th^ ? 216 GENERAL BOTANY which enclosed the female gamete and the empty wall that formerly contained the male gamete disintegrate. When a zygote is newly formed it contains the nuclei derived from the male and female gametes; these unite to form a single nucleus (Fig. 154, A, 5). After a time this nucleus divides to form two daughter nuclei, and each daughter nucleus in turn di- AH Fio. 154. Diagrams showing stages in the history of a zygote of Spirogyra; in section; /, / in surface view. A, zygote just after the union of gametes; gamete nuclei still separate. B, male and female nuclei have united. C, first nuclear division in the zygote, forming 2 nuclei (D). E, second nuclear division, forming 4 nuclei (F). G, 3 nuclei beginning to disintegrate. H, after the disappearance of 3 nuclei. /, J, stages in the germination of a zygote. vides (Fig. 154, C-F). The four nuclei now present in the zygote are similar when first formed, but three of them soon show signs of disintegration and eventually disappear. The fourth nucleus, however, persists and is the sole nucleus present in the mature zygote (Fig. 154, (?, H). The significance of this behavior of nuclei in the zygote will become clear when certain corresponding processes in some of the more complex plants have been dis- cussed (see Chap. XXV). 161. Germination of a Zygote. After the union of the gametes, the color of the zygote contents changes from green to orange-red. Shortly before the zygote is to germinate, its contents again be- come green. The interval between the union of gametes and the germination of the zygote may be a few weeks or a few months, or it may extend from one spring until the next. In germination the heavy outer layer of the zygote wall is broken, and the cell contents, surrounded by the inner layer of the zygote wall, form a short CHLOROPHYCEAE (GREEN ALGAE) 217 tubular outgrowth. The structures typical of a Spirogyra cell (chloroplast or chloroplasts, nucleus, and dense cytoplasm) are visible in this cell that lies partly within and partly without the A broken portion of the wall of the zygote. division of the nu- cleus is followed by a transverse division of the cell (Fig. 154, 7). The daughter cell that is now partly within the zygote wall does not divide, but from the outer daughter cell a new filament is produced by repeated cell division and growth (Fig. 154, J). This filament is similar to the parent filaments. When from any cause a cell that has prepared to function as a gamete does not unite with another, it not infrequently rounds up, secretes a thick wall, and so becomes, except for its somewhat smaller size, identical in appearance with a zygote. Such a resting cell (spore) can germinate in the same manner as a zygote to form a new filament. Thus it appears that any cell of a Spirogyra filament is capable of functioning either as a vegetative cell, as a spore which can grow into a new plant, or as a gamete. DESMIDS 162. Structure. Almost every collection of algae from fresh- water pools or lakes contains members of the group known as desmids. It has been seen that the general course of evolution has been from simple to more complex forms. The filamentous colonies of Spirogyra and of Ulothrix represent, therefore, a more advanced as well as a more complex condition than does the one-celled Chlamydomonas. Desmids, on the other hand, are (chiefly) one- celled organisms whose immediate ancestors seem to have been M^kogyr^ filamentous^ algae closely , .related. They illustrate, therefore, the possibility that evolution may at times be from complex to simpler, instead of from simple to more complex. Most of the thousands of known species of desmids are dis- tinguished from other one-celled green algae by a conspicuous median constriction, each cell thus consisting of two symmetrical half-cells (Fig. 155). The cells of various species differ greaHfy in shape and frequently bear spines or other protuberances, Each half-cell, contains at least one chloroplast, often elaborately lobed, and within each chloroplast are one or more pyrenoids. A nucleus lies in the cytoplasm in the region of the median con- striction. 218 GENERAL BOTANY 163. Reproduction. New individuals are formed by the division of a parent cell into two daughter cells (Fig. 155, (?). Before the cell divides the nucleus divides, each half-cell receiving a daughter ' A FIG. 155. Desraids. A, Closterium. B, Xanthidium. C, Staurastrum. Z>, Mi- crasterias. E, F, Cosmarium. G, division of a Cosmarium cell. nucleus. Nuclear division is followed by a transverse division of the cell in the plane of the median constriction. Each daughter cell &t first consists, therefore, of one half-cell and a portion of the FIG. 156. Staurastrum; stages in the union of gametes.^ De Bary. Adapted from median region of the parent cell. Later, by a growth of the constricted portion, each daughter cell develops a new half-cell. In most desmids the daughter cells become separated from each other as the new half-cells are forming, but in a few species the daughter cells remain united and by repeated division give rise to a filamentous colony. CHLOROPHYCEAE (GREEN ALGAE) 219 154. Gametic Union. Occasionally, when two mature cells come to lie close to each other, their walls break at the median constrictions and their protoplasts function as gametes (Fig. 156), flowing out and uniting with each other to form a zygote. These non-flagellate gametes resemble those of Spirogyra except that they are not differentiated as male and female. When first formed the zygote is naked, but soon after its formation it secretes a thick wall. After a considerable period of rest the wall of the zygote breaks or becomes gelatinized and its contents develop into one, or (by division) into two or four vegetative cells of the form characteristic of the species. OEDOGONIUM i/' 155. Structure and Reproduction. Oedogonium is another unbranched filamentous green alga of frequent occurrence, at- 3 A C D FIG. 157. Oedogonium. A, vegetative cell. B, swarm-spore before liberation, (7, liberation of a swarm-spore. D, young plant produced by the germina- tion of a swarm-spore. B-D after Hirn. tached or forming floating masses in pools and other bodies of quiet water. The cylindrical cells (Fig. 157, A) are joined end to end. Inside the wall of each cell are a layer of dense cytoplasm containing a single chloroplast, a nucleus, and a large central vacuole. The chloroplast has the shape of a hollow cylinder with many irregularly shaped perforations. The large nucleus may lie toward one side of the cell, or may be suspended by cytoplasmip 220 GENERAL BOTANY strands in the middle of the central vacuole much as is the nucleus of Spirogyra. Each cell of a filament may reproduce by division, the subsequent growth of the two daughter cells resulting, as in Spirogyra, in an increase in the length of the colony. As in Spirogyra, too, the num- ber of colonies may be increased by an accidental breaking of the filament. Reproduction of a colony occurs also through the formation of swarm-spores (Fig. 157, Z?, C). The protoplast of a cell withdraws somewhat from the wall, becomes rounded, and develops A a colorless area at one side. circle of flagella is developed at the margin of this colorless area. The protoplast has thus been metamorphosed into a swarm-spore. After the spore is mature, the old wall enclosing it splits transversely at one end, and the spore, moving slowly out through the opening in the wall, swims aw'ay by means of its flagella. After swimming for some time, the spore comes to rest with its flagellate end in contact with some solid body, often a filament of Oedogonium. Soon the spore withdraws its flagella and secretes a wall, and its colorless end becomes modified into a disk-shaped or root-like holdfast. The cell now increases somewhat in length (Fig. 157, D) and then divides transversely into two daughter cells. The lower daughter cell, that with the holdfast, does not divide again; the upper cell by repeated transverse divisions gives rise to a long filament, which becomes free-floating if accidentally broken from its attachment. 156. Gametic Union (Fig. 158). Oedogonium forms gametes of two very different sorts. Any cell in a filament, except the basal cell, is capable of becoming an oogonium, but a cell which so de- velops is always one formed by a recent division. An oogonium becomes somewhat broader than a vegetative cell and spherical or ellipsoid in shape. Its protoplast (the female gamete or egg) shrinks and rounds up entirely within, and free from, the wall. As the egg approaches maturity, a small circular pore may be formed in the oogonial wall. In some species the wall cracks transversely instead of forming a pore. Since the oogonium and the egg are really the same cell, it is hardly necessary to apply both names in Oedogonium. But for the sake of harmonizing the use of terms in this and in some other algae in which an oogonium forms several eggs by division, it is customary to distinguish the egg of Oedo- CHLOROPHYCEAE (GREEN ALGAE) 221 gonium, which is a' protoplast only, from the oogonium, which is the protoplast plus the enclosing wall. Simultaneously with the development of oogonia, certain other cells of the same or of another filament by repeated trans- Antheridium Antherozoid / Oogonium A B FIG. 158. Oedogonium. A, antheridia with antherozoids. B, oogonium in which an egg and an antherozoid are uniting. C, oogonium containing a mature zygote. After Him. verse division form a short series of disk-shaped cells, each of which is an antheridium. The protoplast of each antheridium either becomes a male gamete (antherozoid) or divides to form two antherozoids. Except for their smaller size and their fewer flagella, anther- ozoids are similar in structure to swarm-spores. They are liberated from the walls enclosing them in the same manner as are swarm- spores. In certain species, antheridia are borne on very small, few-celled filaments which are attached to a filament bearing oogonia. An antherozoid swimming in the vicinity of an oogonium responds to a stimulus, exerted probably by a substance diffusing from the oogonium, swims through the pore or crack in the oogonial wall, and unites with the egg. The resultant zygote soon secretes a thick wall which often bears ridges, spines, or otKer "protuberances. 222 GENERAL BOTANY The zygote is eventually liberated by the decay of the oogonial wall. After a period of dormancy it germinates (Fig. 159, A). The zygote wall breaks open and the protoplast by division forms four protoplasts (Fig. 159, B, C), each of which becomes a swarm- spore that may come to rest and develop into a new filament. In the organization of its colony, Oedogonium represents no advance over Spirogyra or Ulothrix. However, in the marked A Oog onium Zygote Wall FIG. 159. Germination of the zygote of Oedogonium. After Juranyi. differentiation of its gametes it presents a condition far in advance of that found in either Ulothrix or Spirogyra. VAUCHERIA 157. Structure and Reproduction (Fig. 160). Vaucheria com- monly forms a green, felt-like mass on damp soil or in shallow water. Each plant-is a sparsely branched thread that may attain a length of several inches but consists of onJy a single cell. Within the wall of this cell is a layer of dense cytoplasm containing small flattened, rounded chloroplasts. Imbedded in the dense cytoplasm are numerous nuclei. A noteworthy feature of the chloroplasts of Vaucheria is the absence^ of pyrenoids and of their accompanying starch granules, the reserve Food being^stpred^ in_the Jorm^ofjoil droplets. The central