Q. 3. Describe the female reproductive organ of Riccia. (2006)
Ans. Female Reproductive Organ of Riccia: -
The female reproductive organ of Riccia is known as archegonium. The mature archegonium is flask shaped structure differentiated into three parts : (i) a short stalk (ii) swollen venter and (iii) a long neck. The neck consists of a single layered tube of 6 - 9 tiers of cells arranged in 6 vertical rows, surrounding a narrow neck canal. The neck canal encloses 4-6 neck canal cells. The apical part of neck is covered by 4 cover cells. The globular venter consists of single layered wall about 12-20 cells in perimeter. It encloses a venter canal cell and a large naked egg.
Development of Archegoinum: -
The archegonium arises from single superficial cell which acts as archegonial initial.
The archegonial initial divides by transverse division forming a basal cell and an outer cell. The lower basal cell remains embedded and divides few times to gve rise the embedded portion of the archegonium. It may develop a short stalk. The upper outer cell becomes the archegonial mother cell. It enlarges in size and divides vertically by the formation of three sucessive ecentric vertical intersecting walls cutting archegonial mother cell into 4 cells. These divisions result in the formation of one median primary axial cell surrounded by three peripheral initial cells.
Each peripheral initial cell divides by radial longitudinal wall to produce six jacket initials, which enclose the primary axial cell. The jacket initials divide by transverse division forming six cells in upper tier and 6 cells in lower tier. The cells of upper tier, called neck initials, divide transversely forming six vertical rows of 6-9 cells. These cells constitute the neck of mature arehegonium.
The cells of lower tier, called venter initals, divide vertically and transversely giving rise the venter of the mature archegonium.
Simultaneously, the axial cell divides by transverse wall forming upper primary cover cell and lower central cell. The smaller upper primary cover cell divides by two vertical walls by the formation of two intersecting vertical walls. It results in the formation of four cover cells. The larger lower central cell divides transversely forming upper primary neck canal cell and lower venter cell.
The primary neck canal cell divides by transverse divisions giving rise to a row of 4-6 neck canal cells. The ventral cell divides transversely forming upper venter canal cell and lower large egg cell. Along with the development of archegonium a few neighbouring cells of thallus divide and grow upward forming an archegonial chamber.
Q.4 Make only labelled diagrams showing the life cycle of Riccia. (1996)
Related Questions -
Q. Draw labelled diagram of life cycle of Riccia. (2009, 12, 13)
Ans.
Q. 5. What do you understand by ‘alternation of generation’? Explain it in the life cycle of Riccia. ( 2011)
Ans. When the sporophyte haploid phase and gametophyte diploid phase comes alternatively after each other in the life cycle of any plant, is called alternation of generation.
Alternation of Generation in Riccia: -
Life cycle of Riccia is started from gametophytic plant body. At the time of reproduction antheridium and archegonium are developed as male and female sex organs respectively which produce antherozoids and egg as male and female gametes. Gametes participate in fertilization. After fertilization diploid zygote is formed. Development of zygote occurs within the archegonium attached with gametophytic thallus.
Zygote is the first cell of sporophytic generation. Zygote forms haploid spores after division and metamorphosis. Haploid spores are first cell of young gametophytic generation. These spores dispersed from sporogonium and germinate in fovourable conditions. After germination they form new gametophytic plants. Thus life cycle of Riccia is completed in two definite phase that comes alternatively. So, a well developed and definite type of alteration of generation is found in Riccia.
Q.6 Among the sporangia of Riccia and Anthoceros which is more primitive and why? (2006)
Ans. The sporangia or sporogonia of Riccia is more primitive than of Anthoceros. Since it has no diploid or sporophytic cells at maturity. The spores are haploid and they are the first cells of gametophytic generation. The calyptra layer is a part of gametophytic. Therefore, morphologically the mature sporangium of Riccia cannot be regarded as sporophyte. Sporogonia of Riccia is entirely dependent on gametophyte.
On Contrary, Sporangia or sporogonia of Anthoceros shows peculier features and a highly developed structure. It is more nearly independent of haploid phase.