Botany and Plant Science

Apospory is the formation of gametophyte directly from sporophyte without meiosis and spore formation. Apogamy is the development of sporophyte directly from gametophytic tissue without fusion of gametes. Parthenogenesis is the development of embryo from egg without fertilisation.

A gametophyte is the haploid multicellular stage in the alteration of generations during the life cycle of land plants and algae. It is produced from mitotic cell division of spores, which are produced by meiosis in sporophytes.

Adventive embryony is also known as sporophytic apomixis, sporophytic budding or nucellar embryony.

The point on the body of the ovule at which the funiculus attached is known as hilum. The ovules are attached to the ovary wall placenta by a slender stalk called funiculus. The anterior part of the ovule is called micropylar end and posterior part is called chalazal end.

Anatropous ovule: A completely inverted ovule turned back 180 degrees on its stalk. Hemianatropous ovule: Ovules are half-inverted so that funiculus is attached near middle with micropyle terminal and at right angles.

Ferns and Cycas are both cryptogams (seedless plants) that reproduce using spores. Their male gametes (sperm) are motile and flagellated (ciliated), requiring water for fertilization. They do not produce seeds, trachea, or cambium.

In Cycas, the vascular bundles in the rachis are arranged in an omega-shaped ring (Ω-shaped). This is a characteristic feature of cycad stems, where the xylem and phloem are organized in a curved, horseshoe-like pattern that resembles the Greek letter omega.

In dicots, the most common type of pollen tetrad arrangement is tetrahedral, where the four microspores are arranged in a three-dimensional pyramid-like structure. This occurs when microspores divide in all planes. Isobilateral is more common in monocots, while linear and decussate are less frequent patterns.

The leaves in Selaginella are ligulate, in that they bear a flap like or finger like projections at the base called the ligule.

Passage cells (also called transfusion cells) are thin-walled endodermal cells found opposite to protoxylem patches. They lack Casparian strips and allow water and minerals to bypass the endodermal barrier, facilitating transport to the xylem. Sclereids are thick-walled supportive cells, and epiblema (rhizodermis) is the root epidermis.

Cycas exhibits polyxylic wood formation where successive cambial rings are produced annually in the pericycle or cortex because the first cambial ring is short-lived. This results in multiple wood cylinders being formed over time, unlike monoxylic (single wood cylinder) or pycnoxylic (compact wood with little parenchyma) patterns. Polyxylic wood is characteristic of certain cycads and some other ancient plant lineages.

Gymnosperms typically lack vessels in their xylem tissue, relying primarily on tracheids for water and mineral transport. Tracheids are the fundamental water-conducting cells in gymnosperms, while vessels are a more advanced evolutionary development found predominantly in angiosperms. Xylem fibres and xylem parenchyma are present in gymnosperms as they serve structural and storage functions respectively.

Dicot stems possess conjoint, collateral, endarch and open vascular bundles. 'Conjoint' means xylem and phloem are together, 'collateral' means they're on the same radius, 'endarch' describes protoxylem facing inward, and 'open' means cambium is present for secondary growth. Monocot roots and stems have different arrangements - scattered vascular bundles in stems and radial arrangement in roots.

Pteridophytes (ferns and their relatives) reproduce via spores, have vascular tissue (xylem and phloem), but lack flowers and seeds. Thallophyta lack vascular tissue, bryophyta lack vascular tissue, and gymnosperms have seeds.

One sperm fertilises the egg cell and the other sperm combines with the two polar nuclei of the large central cell of the megagametophyte.