Polytrichum Commune: External Characteristics

PART II. BRYOPHYTA. A. MUSCI. POLYTRICHUM COMMUNE, L. A. GENERAL EXTERNAL CHARACTERS. I. Observe in well-grown specimens of this Moss taken in spring or early summer 1. The erect stem, 1 which may attain a considerable length, branching but rarely : 2. The 1 leaves, of relatively small size, and simple form ; their arrangement is on a complicated plan (see below) : at the base of the stem note A 3. dense mat of rhizoids of brown colour. At the apex of some specimens will be found merely a bud, composed of young leaves of the vegetative type ; other specimens will bear at their apex 4. Perigonia, or perichaetia ; cup-like rosettes of leaves, which assume a bright reddish or orange colour, 1 Though the terms "stem" and "leaf " are used here, it must be distinctly borne in mind that the members thus named, being parts of the oophore generation are not homologous with, but at most only analogous to the stem and leaf in vascular plants, which are parts of the sporophore generation. B 2 PRACTICAL BOTANY. and protect the antheridia ; other specimens again may bear at their apex 5. The mature sporogonium or spore-capsule, of which the head or theca is supported on a long stalk, or seta. Note in specimens which are not too ripe a. The calyptra, a dry fibrous hood, covering the apex of the sporogonium : beneath this is I. The lid-like operculum with its terminal beak : this lid may be easily detached, disclosing c. The thin membranous, and transitory epiphragm. d. The peristome, a fringe of fine fibres, which by their hygroscopic movements assist in scattering the spores. e. At the base of the theca observe a swelling called the apophysis. /. By carefully removing the leaves from the apex of a plant bearing a sporogonium, it may be seen that the base of the seta is enveloped by a closely fitting sheath, the veil or vaginula, the origin of which will be explained later (page 9). Observation of the external characters of a simpler Moss, e.g. Funaria hygrometrica, will give in the main similar results to those above described for the larger Polytrichum. B. MICROSCOPIC INVESTIGATION. Oophore Generation. II. Cut transverse sections of a mature stem of Polytrichum : mount some in glycerine, others in Schulze's solution or in iodine solution : examine them first under a low power, and observe in those mounted in glycerine MOSS. OOPHORE. 3 1. The outline of the section, which is usually more or less clearly triangular. 2. The dense reddish-brown band of peripheral sclerenchyma, which passes over gradually into A 3. broad, thinner-walled band of tissue, which may be termed the cortex : this finally surrounds A 4. central mass of firm, yellow-walled tissue. Examine these several tissues in detail, under a high power, and observe that 1. At the extreme outer limit is a thin cuticle, with small and irregular outgrowths: there is no clearly defined epidermal layer. 2. The peripheral sclerenchyma consists of cells with clearly stratified red walls, which are of such thickness as almost to obliterate the cell-cavity. 3. The broad band of tissue of the cortex (3, above) has relatively thin, yellowish or colourless walls, and protoplasmic contents with starch granules, and globules of oil (compare sections treated with iodine). 4. The central mass of tissue (which may be com- pared to a vascular bundle) is not sharply limited from the cortex : it consists of A a. peripheral, small-celled, and thin-walled portion, the walls of which do not stain blue, but light yellow with Schulze's solution. A I. central, thick-walled part, without cell- contents : the thick walls stain dark brown with Schulze's solution: the elements are often divided by delicate septa, which are not stained by Schulze's solution. This tissue may be compared to the xylem of true vascular plants. B2 4 PKACTICAL BOTANY. Here and there small groups of tissues similar to the above may be seen in the cortex ; these are the strands which enter the stem from the leaves, and pass inwards towards the central bundle. III. Cut median longitudinal sections of the mature stem of Polytrichum : mount as before, and note that the peripheral sclerenchyina consists of elongated prosenchymatous elements, while the cells of the massive cortex are of a parenchymatous form. The elements of the central strand are elongated, and their lateral walls smooth, without pits : they are separated one from another by thin oblique septa. Observe that where the median plane of a leaf has been cut through longitudinally, a strand of tissues similar to those constituting the central strand may be traced, passing obliquely through the cortex towards the central strand. By cutting transverse and longitudinal sections of the apex of the stem of Polytrichum, or Funaria, it may be ascertained that there is in each case a single apical cell of tetrahedral form ; that segments are cut off successively from the three sides ; and that one leaf originates from each segment : in the transverse sections it may, however, be seen that in both plants the angle of diver- gence between the successive leaves (and similarly between the successive segments) is larger than one-third : thus the leaves form three parastichies, and this will account for the apparent complexity of their arrangement in these plants. In Fissidens the arrangement is in two longitudinal rows, or orthostichies, and there is a bilateral wedge-shaped apical cell. In Fontinalis there is a tetrahedral apical cell, but the divergence of both segments and leaves is one-third. IV. Strip off a few mature leaves : mount one of them in water, with the upper surface uppermost, and observe under a low power that the narrow, linear upper portion is marked on its upper surface by MOSS. OOPHORE. 5 longitudinal strise (the lamellae), and has a minutely serrated margin : the basal portion of the leaf, which is closely applied to the stem, is broad, but thin and membranous, and is not marked by longitudinal striae. V. Cut transverse sections of leaves : this may easily be done by holding the terminal bud of a mature plant between pieces of pith, or by embedding in paraffin, and then cutting transverse sections of the whole bud. Mount all the sections as before, and examine first with a low power. Neglecting the almost circular transverse sections of the stem, recognise 1. Those transverse sections which have passed through the sheathing basal portions of the leaves : these may be readily distinguished by their broad lateral wings, only one layer of cells in thickness. 2. Those which have been taken from the upper part of the leaf : these may be distinguished by their more bulky appearance. Having recognised these sections, put on a high power and examine them in detail : 1. In the section of the sheathing base of the leaf observe a. The two lateral wings, consisting of a single layer of cells, with thickened outer walls, and but little chlorophyll. Z>. The more bulky central portion consisting of An i. irregular layer of superficial cells (epidermis ?) with thickened outer walls, covering both upper and lower surface : beneath these are ii. Bands of sclerenchyma, in which the lumen is almost obliterated. iii. Within these lies a "vascular bundle " consisting 6 PKACTICAL BOTANY. of elements essentially similar to those composing the central bundle of the stem. 2. In the sections of the upper part of the leaf note that the arrangement of the tissues is for the most part similar to that in the above sections, but rather more bulky, while opposite each of the cells at the upper surface is seen to be attached a series of three to five chlorophyll-containing cells, which represent transverse sections of those longitudinal plates or lamellae above observed on the upper surface of the leaf, under a low power. It is now obvious that these chlorophyll- containing lamellae are separate laterally from one another: they constitute the chief assimilating tissue of the plant. For comparison with, the above, a Moss of simpler type may be taken, e.g. Funaria Jiygrometrica. As before there is an erect stem, with a mat of brown rhizoids at its base : it bears a number of leaves of somewhat variable form, more or less widely ovato- lanceolate. Mount a single leaf in water and examine under a low power : note the clearly marked midrib, terminating in the acuminate apex, the thin lateral portions, consisting of only a single layer of cells containing chlorophyll, and bounded by an entire margin. Gut transverse sections of the stem, and mount in weak glycerine or glycerine jelly : on examination under a low power it will be seen that the stem is of much simpler structure than that of Polytrichum : the peripheral tissues have brown walls, but they are not thickened to any marked degree : at the centre is a strand of thin-walled, small-celled tissue : the peripheral tissues usually contain chlorophyll. Sexual Organs. VI. Take a mature antheridium-bearing axis of Polytrichum, and dissect it with needles in a watch- MOSS. OOPHORE. 7 glass, keeping all the parts detached. Examine them carefully with a lens, and observe the following categories of organs 1. The perigonial leaves, which are widened laterally into very broad membranous wings, with a clearly defined, central midrib. 2. The white, club-shaped antheridia. 3. The paraphyses, which will often be found associated with the antheridia : some of them are simply filamentous, others are more or less clearly spathulate. VII. Cut median longitudinal sections of a male axis : mount in weak glycerine, and with a low power recognise the several organs above described, and their relative positions : note especially the antheridia in the axils of the perigonial leaves. Observe under a high power the structure of a single antheridium : it consists of a short stalk, and a club-shaped body, composed of a wall (i.) a single layer of cells in thickness, and (ii.) a central mass of cells of more or less clearly cubical form : these are the mother-cells of the antherozoids. VIII. Take fresh antheridium-bearing specimens of Polytrichiim after some days of dry weather (or keep them rather dry for some days, carefully preventing any access of water from above) : squeeze one of them between the finger and thumb : the antheridia will thus be easily forced from their position, and may be mounted in water. If they were properly mature, it may then be seen that on contact with water the antheridia burst, and the mother-cells escape, aggregated in a mass. In each cell of this mass a spiral filament PKACTICAL BOTANY. may be seen, in active movement : it may be seen to escape ultimately, owing to mucilaginous swelling of the wall of the mother-cell, as a free antherozoid of spiral form, having two cilia. Antheridia may be obtained on Funaria hygrometrica at almost any period of the year : they are borne on special branches as in Polytrichum, but are surrounded by a less conspicuous perigonium : these axes are usually shorter than those which bear the female organs. IX. Take a sod of Funaria 1 with no sporogonia as yet visible upon it, but which bears antheridia : these will be situated at the apices of the shorter axes : many of the longer axes will appear to be terminated by ordinary vegetative leaves, and it is on these axes that the archegonia may be found. From such buds, after hardening in alcohol, cut median longitudinal sections : if not transparent enough treat with dilute potash solution, and mount in weak glycerine : subsequently the sections may be transferred to glycerine jelly. Examine first with a low power, when the usual arrangement of axis and leaves may be observed ; between the youngest leaves an archegonium (or several) may sometimes be detected. If mature, it will be seen to be a flask-shaped organ, seated on a short massive stalk : it consists of 1. An elongated neck, more or less contorted, com- 1 Since Funaria hygrometrica produces sporogonia at all times of year, and is very 'common, while Polytrichum commune is reproduced sexually only in the spring and early summer, it will be convenient in most cases to use the former in examining the archegonia. In either case, however, it is a matter of some experience and expenditure of time to get a good series of preparations illustrating the development and structure of the archegonium, and the early stages of the production of the sporogonium. MOSS. SPOROPHORE. 9 posed of a single layer of cells arranged in four to six rows : these surround a central canal, which is filled with mucilage at the time of fertilisation, but before maturity there may be seen within it a series of canal cells. A 2. lower, enlarged ventral portion, consisting of two layers of cells, which constitute the "wall, and inclose a central space, in which may be seen the naked spherical oosphere or ovum, and above it (up to the period of maturity) the smaller ventral canal cell. Similar sections, similarly prepared from rather older speci- mens, may show as the first results of fertilisation that the neck of the archegonium turns brown and withers, while the wall of the ventral portion and the stalk show considerable increase in bulk, and frequent cell-divisions. Meanwhile growth and celldivision take place also in the fertilised ovum, resulting ultimately in the development of the sporogonium : the growth of the wall of the archegonium keeps pace for some time with that of the young sporogonium, completely inclosing it, but as it increases towards maturity, the wall of the archegonium is ruptured transversely about half-way up : the apical part is carried upward by the growing sporogonium, as the calyptra which covers its apex, while the lower portion remains as an investment of the base of the seta, and is called the veil or vaginula. Note especially in young sporogonia the two-sided, wedgeshaped apical cell, with segments cut off from either side. Sporophore Generation. The Sporogonium. X. Having noted the external characters of the sporogonium of Polytrichum, as above described, cut transverse sections of the mature seta : mount in 10 PKACTICAL BOTANY. glycerine or glycerine jelly. Being a cylindrical organ the transverse section is circular. Note 1. The superficial layer of cells with a definite cuticle and thick yellow walls. A 2. band of brown sclerenchyma, which graduates internally into A 3. thin-walled parenchyma with large inter- cellular spaces, and containing chlorophyll. 4. Centrally is a strand of denser tissue without intercellular spaces. XI. Cut median longitudinal sections of the base of the seta, which is inserted on the apex of the Moss-plant or oophore : mount as before, and note in the upper part of the seta the superficial layer, brown sclerenchyma, thin-walled parenchyma, and central strand, as above described. Following the seta down to the base, it will be seen that the cuticle and brown sclerenchyma stop short, and are replaced by thin-walled parenchyma with plentiful protoplasm ; this tissue of the sporophore is in close connexion with the inner surface of the vaginula, which belongs to the oophore generation. With this close physiological connexion of the sporophore and oophore in the Moss, compare the connexion by means of the foot in Ferns. Similar sections may be prepared, with similar results, from plants of Funaria, but there will be greater difficulty in this case, owing to the smaller size of this Moss. XII. Passing now to the apex of the sporogonium of Polytrichum remove the calyptra : mount it in water or weak glycerine, and examine under a low power. It consists of dry, often branched, hypha-like filaments, loosely matted together : the neck of the MOSS. SPOROPHORE. 11 archegonium may often be recognised at its extreme apex. The calyptra of Funaria may be treated in a similar way : here the brown neck of the archegonium is clearly seen, while the body of the calyptra consists of a continuous tissue, a single layer of cells in thickness. XIII. It will be found convenient to take first the capsule of one of the simpler Mosses, e.g. Funaria, and subsequently to proceed to a more complicated example, e.g. Polytrichum. After noting the oval form, and the obliquely placed operculum, embed capsules of Funaria, which have been hardened in alcohol, in paraffin : and cut median longitudinal sections : mount in glycerine or in glycerine jelly, and examine first with a low power. N.B. It will be well to select young capsules of such age that the peristome (seen through the operculum) shall show a pale yellow tinge : later it assumes a dark yellow or orange colour, and in such cases the spores would be almost mature : the structure of the whole capsule would accordingly be more difficult to understand than in the younger specimens. Observe 1. The seta, which widens out gradually into 2. The theca or capsule, without any clearly marked basal swelling, or apophysis : at the apex of the theca observe that the section has traversed 3. The operculum or dome-like lid : beneath this is 4. The yellow or orange peristome : in the lower part of the capsule note 5. The outer wall consisting of a clearly marked epidermis, and beneath it some three or four layers of thin-walled parenchyma. 12 PKACTICAL BOTANY. 6. The air-space, traversed obliquely by filaments of thin-walled cells, suspending a central mass con- sisting of 7. The spore-sac, which will be seen to be composed of two or three layers of thin-walled cells. A 8. layer of spore-mother cells, recognised by their dense protoplasmic contents. 9. The central mass of thin-walled tissue of the columella, which extends upwards into the concave operculum. By careful observation of longitudinal sections under a high power the following points may be verified : 1. That stomata occur in the epidermis towards the base of the sporogonium. This may be confirmed by cutting tangential sections from the base of the sporogonium, in which the stomata, with their two guard-cells, may be clearly seen in surface view. 2. That immediately above the upper limit of the air-space there is a band of brick-shaped cells, elongated transversely, and with pitted walls : these are not derived from the epidermis but from the subjacent layers, and serve to connect the peristome with the outer wall of the theca. Closely above this band the tissues are again more delicate, and it is here that the rupture of the capsule takes place, by which the operculum is set free. Above this more delicate zone is 3. The slightly projecting lower lip of the operculum, at which point the epidermal cells are thickened, and of peculiar form, constituting the annulus. Beneath the operculum, which consists of the superficial layer of epidermis together with two or three layers of thin- walled tissue, lies MOSS. SPOROPHORE. 13 A 4. layer of cells with peculiar yellow or brown thickening of the walls : this gives rise to the peristome. Returning to the spore-forming layer, it may be verified under a high power i. That it consists at first of a single layer of cells (the archesporium) : to ascertain this for certain it may be necessary to cut sections from sporogonia of various ages. ii. That the cells of this layer divide repeatedly ; and ultimately, by division of each of the resulting sporemother cells into four, the spores are produced. Mount some mature spores in water : they will be seen to be of spherical form, with smooth walls, and granular, oily contents. XIV. In order better to understand the structure and origin of the peristome, cut off transversely the whole orange-coloured tip of a sporogonium of Funaria, and mount it with the apex uppermost in weak glycerine : observe under a low power i. The contorted brown teeth of the peristome, sixteen in number : these are easily seen through ii. The more transparent operculum, which covers them : by focusing carefully downwards iii. The much thickened cells of the annulus may also be distinctly seen. XV. Cut thin transverse sections of the operculum of a still unripe sporogonium : in those which pass imme- diately above the annulus, observe at the periphery a. The thickened epidermis, and beneath it two or three layers of thin-walled cells : these together with the epidermis constitute the operculum : then follows 14 PRACTICAL BOTANY. A I. layer of cells with peculiar thickening bands on the inner and outer walls : these bands separate, by rupture of the thinner parts of the walls, as the inner and outer peristome, which thus consists when mature of ribbands of cell-wall, and not of complete cells. The above points may with advantage be confirmed by observations on mature sporogonia. For comparison with the sections above described, transverse and longitudinal sections may be cut from young sporogonia of Funaria of various ages, and the development of this peculiar organ may thus be traced. For details of description see the text-books ; especially Goebel's Outlines, Eng. Ed., p. 186. XVI. Longitudinal and transverse sections may also be made from the sporogonium of Polytrichum commune, and this may be treated as above directed. The chief differences from Funaria will be as follows 1. The clearly marked apophysis. 2. The presence of two concentric air-spaces, with the spore-sac between them. 3. The more bulky operculum. 4. The proportionately smaller peristome, consisting in this case of bundles of prosenchymatous cells. 5. The presence of an epiphragm, below the oper- culum, and connecting the teeth of the peristome temporarily together. 6. The less clearly marked annulus. XVII. Scatter spores from the ripe sporogonium of Funaria or Polytrichum over moist soil, and keep them at a moderately high temperature, under a bell-glass, for a few days. The surface of the soil will soon be seen MOSS. PEOTONEMA. 15 to be overgrown by numerous fine green filaments. Having carefully removed some of these with a needle, and having washed the soil from them, mount them in water, and examine them under a high power. Note i. The dark-coloured exospore, which may be found still attached to the filaments after they have attained a considerable length. ii. The fine filamentous protonema resulting from out-growth of the endospore : observe especially the septa, which are often oblique ; the branches, usually arising immediately below a septum : the various development of these branches, either a. As relatively thin filaments with brown cell-walls, and no chlorophyll : these are the rhizoids, and they penetrate the soil. &. As relatively thick filaments, with colourless cell- walls, and chlorophyll : these constitute the true protonema. c. As solid buds, which are usually situated at the base of one of the branches such as a or b : in these solid buds of various ages may be traced the successive stages of development of the Moss-plant, which is thus produced as a lateral bud on the protonema. Cultures of protonema, showing all the most important cha- racters above noted, may be obtained at any time of year by cutting fine sods of Funaria, inverting them under a bell-glass, and growing them in moist air and at a moderate temperature for two or three weeks. In the case of Fwtaria the protonema may under these circumstances be induced to form terminal unicellular gemmae, which are easily detached from the parent protonema, and by germinating reproduce the plant in a vegetative manner. It will also be found possible, by culture of detached -leaves 16 PRACTICAL BOTANY. and portions of the stem of the Moss-plant on moist soil, and under other favourable conditions, to induce a formation of protonemal filaments by direct outgrowth of cells of those parts. SPHAGNUM (the Bog-Moss}. XVIII. Take a plant of any native species of Sphagnum, and observe with the naked