Lecture 4: Hornworts PDF

Lecture 4 : Hornworts A hornwort is a flowerless, spore-producing bryophyte. It is characterised by its spores which are typically produced in a tapering, horn-like or needle-like capsule that develop from a flattish, green sheet. All bryophytes have a gametophyte stage and a sporophyte stage and i...

Lecture 4 : Hornworts A hornwort is a flowerless, spore-producing bryophyte. It is characterised by its spores which are typically produced in a tapering, horn-like or needle-like capsule that develop from a flattish, green sheet. All bryophytes have a gametophyte stage and a sporophyte stage and in hornworts the gametophyte consists of a flattish, green sheet called the thallus. The male and female gametes (sperm and eggs) are produced on the thallus and a fertilized egg will develop into a sporophyte. It is the sporophyte that has a tapering horn-like or needle-like structure. The English word ​wort means ​small plant and it appears in plant names such as liverwort, pennywort and bladderwort. Hornwort, therefore, can be best described as a “small, horned plant”. The hornwort gametophyte Dendroceros​ sp. a hornwor​t As noted above, the hornwort gametophyte is an often lobed or wrinkled, greenish sheet (or thallus). It may be strap-like (and branched) or grow as a rosette, depending on the genus. The thallus is typically dark green (though light green to yellowish in ​Notothylas)​ and is typically many cells thick, though the genus ​Dendroceros is an exception. In that genus the thallus is strap-like (and branching) and each strap has a thick, central midrib with thin lateral wings, which are often wavy or ruffled. The thallus is attached to the substrate by smooth ​rhizoids​, which are anchoring structures that are superficially root-like but without the absorptive functions of true roots. Rhizoids grow out from the underside of the thallus. In a hornwort thallus there are from one to a few chloroplasts per cell, most commonly just one, large chloroplast per cell (though there may be up to 12 per cell in the genus ​Megaceros​). Within photosynthesizing plants, chloroplasts are the sub-cellular structures that contain chlorophyll. In mosses and liverworts there are numerous chloroplasts per cell. Growing tips are covered by mucilage which is produced by the surface cells. Hornworts don't have the slime papillae that are found in many liverworts, but appear to be able to produce mucilage in almost any cell. Within the thallus there are mucilage-containing cavities, formed by the breakdown of many cells into mucilage. As the thallus ages some of these ​mucilage cavities may dry out, resulting in air-filled cavities. The mucilage cavities are connected to the exterior via openings called ​slime-pores or ​mucilage clefts​. The slime-pores are on the thallus underside and through them ​Nostoc (a cyanobacterium) can enter the mucilage cavities. Once inside a mucilage cavity the ​Nostoc multiplies and forms a symbiotic association with the hornwort. Nostoc provides the hornwort with nitrogen and gets carbohydrates in exchange. All hornworts have these ​Nostoc cavities. They are best developed in the genus ​Dendroceros​, where they are readily visible to the naked eye as blackish dots. In other cases you may need a hand lens to be able to see them for certain. Within a ​Nostoc cavity, outgrowths from the hornwort thallus penetrate the cyanobacterial colony. There is little or no differentiation within a hornwort thallus. That is quite different to the case in the thalli of the ​COMPLEX THALLOSE LIVERWORTS​, where the cells in different layers within the thallus have different functions. Hornworts also lack the scales that are present on the undersides of the thalli of complex thallose liverworts. The hornwort sporophyte A single thallus produces many sporophytes and a hornwort sporophyte is typically an erect, elongated, tapering, horn-like or needle-like structure. It is often several centimetres long (occasionally over 10 cm in length), though in the genus ​Notothylas the sporophytes are always small, often less than five millimetres long. So, rather than horn-like or needle-like, a ​Notothylas sporophyte is short and stubby - often slightly arched and so somewhat banana-like in shape. Also, the ​Notothylas sporophytes are not erect but are roughly parallel with the thallus or angled upward a little. Regardless of the shape, the entire structure is a spore capsule. There is no capsule-on-a-stalk sporophyte that you find so commonly with mosses and liverworts. The sporophyte (except for the genus ​Notothylas​) is constantly growing from a point just above its base. As new cells are added there, the existing sporophyte is naturally pushed out a bit. In theory this process of cell addition and sporophyte growth could continue indefinitely. In practice it does come to an end when conditions become too poor for the gametophyte. For example, if it becomes too dry, or the gametophyte is overgrown. Though the sporophyte contains chlorophyll and does photosynthesize, it still depends heavily on the gametophyte for carbohydrates. If the gametophyte suffers, sporophyte growth may stop. Phaeoceros​ sp. with sporophytes Given the manner of sporophyte growth it's clear that the youngest cells are near the base and the oldest are at the top. The spores mature within the growing sporophyte and the first to mature will of course be those at the top. The spores a little lower down will mature a little later, those further down will mature later still and so on. Amongst the mature spores you will also find pseudo-elaters. These are tiny, filamentous structures that superficially resemble the spiralled ELATERS that are a feature of the capsules in many liverwort species. In the photo (right) the sporophytes are brown in the upper areas and green below. The green areas are those where spores have not yet reached maturity. None of the sporophytes in that photo have yet opened but here you can see a number of open sporophytes. A sporophyte splits along its length (from the top down) to release the spores over an extended period and a sporophyte will often split along two lines. Those lines run the length of the capsule and are opposite one another. The splitting starts at the top of the capsule and moves down, keeping pace with the maturing spores. This process splits the sporophyte into halves - but with the two halves usually remaining joined at the very top. In that case we get two slits, on opposite sides of the sporophyte, that lengthen over time. Equally often the sporophyte splits along one side only, so creating an expanding slit along just one side of the capsule. You will see that in ​Dendroceros and ​Megaceros sporophytes. As you might expect by now, the genus ​Notothylas is an exception to the general rule about the capsule splitting in two. ​Notothylas sporophytes may split in this way, but may also rupture irregularly or the capsule walls may simply decay. Reproduction of Hornworts Hornworts’ reproductive cycle occurs in an alternation of generations, in which a gametophyte creates a plant body (thallus) in the sexual generation, and the sporophyte produces a spore containing capsule in the asexual generation. In the sexual generation, hornworts’ gametophytes are flat and as small as 1 to 2 centimeters (0.4 to 0.8 inch) in diameter. The thallus has hornworts’ sexual organs, the ​male antheridium and female archegonium, on its top surface (whether the male and female organs occur on the same plant varies by species) and rhizoids on the bottom, which secure plants to soils. The hornworts’ gametophytes resemble a clump of small leaves. Occasionally, the cyanobacteria Nostoc can be found in holes in the thallus, where it fixes ​nitrogen to exchange for carbohydrates. Hornworts differ from liverworts in that hornworts lack cellular oil bodies and have mucilage instead of air chambers. During the asexual generation, the sporophyte relies on the gametophyte for food and moisture, remaining connected to it during its life. Water is essential for the transport of sperm from the antheridium to the archegonium, where fertilized eggs become sporangia. From a basal sheath on the thallus, the sporophyte creates a slender, hornlike cylinder, which can be as high as 12 centimeters (4.75 inches) and gives the hornwort its common name. Filled with spores, the sporangium elongates and gradually splits open from top to base, scattering spores. Internally, horn tissue continues to divide to produce thousands of spores. Other cells, called pseudoelaters, help spread spores, which germinate to begin the cycle again. Hornwort spores can survive as long as one decade in soil. Hornwort reproduction can also be achieved vegetatively if the thallus is fragmented. Physically, hornworts resemble liverworts. They subtly differ from liverworts by having a place on the sporophyte’s base, a near-basal meristem, where cells continue to divide and grow during the sporophyte’s life. Hornworts also have a tissue column on the capsule, called a columella. Hornwort gametophytes can be differentiated from those of liverworts and ferns by examination under a microscope to detect one large chloroplast in each hornwort cell. This chloroplast sometimes surrounds the ​nucleus​ and contributes to the color and translucency of hornworts. Magnification also reveals the mucilage between cells, instead of air, where cyanobacteria thrive. Hornwort sporophytes also deviate from those found in mosses and liverworts because they exist even after the gametophyte dies.