Tag: biology

The archegonium which develops from a single superficial cell of prothallus is the archegonial initial found just back of the apical meristem. The archegonia  are sessile and embedded. The archegonial initially divides to give 4+4 cells which make up the neck. The neck has a single neck canal cell.

Strobilus is called the fertile branch, because it bears the reproductive structures such as microsporophylls and megasporophylls.

In Dryopteris, the neck consists of a wall or jacket layer of 4 vertical rows of sterile neck cells. The neck cells enclose a central neck canal. They contains a single, long neck canal cells with two nuclei.

In older plants, the branches bear reproductive structures at their tips. They are called as strobilus or spondiferous spike. The arrangement of micro and megasporophylls in the strobili of different species is variable. The strobili of S.oregana are among the longest in the flora and they often show several novel features. In S.kraussiana, there is only one megasporangium at the base and the rest of the sporophylls bear microsporangia.

Selaginella and Salvinia are considered the first plants to exhibit a significant step toward the evolution of seed habit because embryo develops in female gametophyte which is retained on parent sporophyte. It occurs because Selaginella and Salvinia both are heterosporous, i.e., these plants produce two kinds of spores, macro (large) and micro (small) spores. In the heterosporous species, the female gametophyte remains on the parent sporophytes for variable periods. The development of the zygotes into young embryos takes place within the female gametophytes. This event is a precursor to the seed habit (tendency towards seed formation) considered an important step in evolution. Hence, heterospory leads to seed habits in plants.

The presence of Ligule - a small tongue shaped appendage on the adaxial side of the leaf near the leaf base is a distinctive feature of the Selaginella. This consists of elongated secretory cells the glossopodium which tends to keep the sporangium wet. Another distinctive feature is the cones or strobilus on which the sporophylls are present.

Dryopteris Filix-mas (Linn.) is commonly known as male-shield fern, because they have a shield-shaped protective covering for their spore containing structures. 

Heterospory is a phenomenon in which two kinds of spores are borne by the same plant. These spores differ in size. The smaller one is known as microspore and the larger one is known as megaspore. The microspore germinates to form the male gametophyte and the megaspore germinates to form the female gametophyte. Heterospory evolved first in pteridophytes such as Selaginella and Salvinia. Hence option B is correct.

Under dry conditions, the xerophytic species of Selaginella roll into brown balls but remain uprooted. This phenomenon is called as a cespitose habit. But during moist conditions, the brown balls become green and unroll again. Therefore, these plants are called as resurrection plants. e.g. Selaginella lepidophylla and Selaginella bryopteris.

Peritichious flagellation is found in Escherichia coli where the flagella are projecting in all directions;
The living fossil plant is the Ginkgo biloba.Largest perennial alga is the Macrocystis pyrifera.
Smallest flowering plant is Wolffia arrhiza and belongs to the genus WolffiaRhizophore - the downward-growing leafless dichotomous shoots is a characteristic feature of the genus Selaginella. Thus, the correct answer is option D.