In this article we will discuss about the sexual and asexual modes of reproduction in riccia with the help of diagrams.
Sexual Reproduction in Riccia:
Most of the species of Riccia are monoecious. Antheridia and archegonia occur on the same thallus. However, some species (R. curtisii and R. discolor) are dioecious. In R. curtisii two spores of a tetrad develop into female thalli and two develop into male thalli.
The sex organs are formed in acropetal succession; the younger ones are produced near the apex and the older ones are pushed behind. On monoecious thalli antheridia and archegonia are mixed and develop intermittently. Serial sections through a thallus reveal all stages of development.
Origin of a sex organ can be traced to a superficial cell. The initial cell, antheridial (Fig. 2.3A) or archegonial (Fig. 2.4A) can be identified due to its upward growth and dense contents, in relation to neighbouring cells.
A mature antheridium has a short stalk (Fig. 2.3A), which attaches it to the base of antheridial chamber, and an elongated body with a flat base and conical apex. In this sac-like structure is a mass of androcytes enclosed by one-cell thick wall or jacket of the antheridium.
If water finds its way into the antheridial chamber the apical cells of antheridial jacket enlarge by assimilating water and finally break open releasing a mass of androcytes. The androcytes in fact ooze out of antheridia and metamorphose into antherozoides to reach the archegonia, either by swimming in a surface film of water or are splashed by rain drops. In the absence of water the androcytes are retained in an antheridium.
Metamorphosis (Fig. 2.3B-D) of an androcyte into an antherozoid is a matter of a few to many minutes. It begins with the appearance of an extra-chromosomal body – blepharoplast – at the periphery of the cell. With the elongation of blepharoplast the androcyte becomes circular.
Along with these changes the nucleus also assumes a crescent-like shape, of a narrow and a broad end, and comes in close contact with the blepharoplast. At the anterior end of this body appear two long flagella. A small part of cytoplasm which is not utilized in the formation of flagella /cilia remains attached to the posterior end of the antherozoid.
Archegonium is a long-necked, flask-shaped structure (Fig. 2.4F), attached to the thallus by a short stalk. The swollen basal portion of the archegonium is venter, it encloses an egg and a venter canal cell. The neck is a long tubular structure, enclosing a narrow canal. The archegonial jacket consists of six tiers of elongated cells arranged in six vertical rows.
On the upper part of the neck are four specialized cover cells which are of larger diameter than other cells of neck and help close the neck canal. The canal is made up of four neck canal cells.
Prior to fertilization, the neck canal cells and the venter canal cell disintegrate forming a mucilaginous mass. This mass imbibes water and swells up, exerting pressure on the cover cells to separate, and the canal opens up to the venter. The mucilaginous mass also helps in attracting spermatozoids swimming in a surface-film of water.
The attraction is a chemotactic phenomenon, guided by proteins and inorganic salts present in the mucilage. Under the orienting influence of chemotactic substance/s present in the canal of an archegonium the spermatozoids are guided right up to the egg.
When a spermatozoid is able to fuse with the egg the resulting structure is a zygote. Following gametic union zygote secretes a wall and enlarges itself to occupy the entire lumen of venter. Fertilization also stimulates division of venter cells which become two-cell thick.
Zygote (Fig. 2.5A) is the first cell of the sporophyte. The sporophyte, sporogonium of Riccia is simplest in organization; it is a globular capsule, without a seta and foot. The sporogonuim produces a large number of spores.
A spore (Fig. 2.5G) can be resolved to have three layers. The outermost (perispore) – perine is thin and mucilaginous but strongly cutinized. Following it is a tough outer spore coat (exine) in three concentric layers. The inner layer (intine) is thin and homogenous.
The exine is variously ornamented; showing a complete reticulate sculpture, an incomplete to irregular reticulum or even tubercles. Distinctive ornamentation patterns are helpful in delimiting species. Each spore has two convex distal faces and opposite it are three flattened faces forming a pyramid.
Dispersal of spores, in the absence of any mechanism, is possible on disorganization of capsule wall and decay of surrounding thallus tissue. In this process the spores remain inside the thallus for a year or so. The spores are finally dispersed by air currents or more commonly by splashing rain drops, to germinate and give rise to gametophytes.
Germination of spores is possible in moist environment. A spore swells by absorbing water and from its distal face arises a germ tube (Fig. 2.5H). At the apex of germ tube initiation of thallus is possible when one of the four cells functions as an apical cell with two cutting faces. It cuts off a number of cells resulting in a multicellular thallus (Fig. 2.5J). Rhizoids emerging from the multicellular thallus (Fig. 2.5K) fix it to the soil.
Asexual Reproduction in Riccia:
Along with the completion of sexual cycle which brings in new gene combinations, thalli in some species such as R. discolor (Fig. 2.1E) and R. billardieri (Fig. 2.1F) develop special circular structures, the tubers, for asexual reproduction.
These tubers are commonly at the apices and remain buried in the soil and develop copious rhizoids. The tubers help to perennate the plant under adverse conditions and rejuvenate to form a new thalli on return of favourable conditions. In this way, nature helps to clone a particular cell line.
