Study Notes on Biota (With Diagram) | Evergreen

The below mentioned article provides study notes on Biota.

Biota, a common ornamental plant of Indian gardens, is a monotypic genus of Cupressaceae and represented by only B. orientalis. It is often confused with and was once included under Thuja as one of its species.

On the basis of some morphological peculiarities, Endlicher (1847) gave Biota a generic rank, and the same was supported by Pears (1937) on the basis of his studies on wood anatomy, Bucholz (1948) on the basis of his studies on polyembryony and Martin (1950) on the basis of his studies of embryology.

Biota is an evergreen tree with a deep penetrating tap root system and numerous green branches. Mature plants reach up to 10-12 metres in height. The older branches remain covered with smooth brownish bark.

Young plants bear vertical lateral branches which become horizontal in the later conditions. Persistent foliage leaves are present on the adult plants. The leaves are closely appressed (Fig. 11.51 A), acute and decussate. Each leaf remains fused with the stem along one-third of its length.

Anatomically, young stem remains covered by a thickly circularized epidermis, a well-developed cortex and a ring of vascular bundles which are conjoint, collateral, open and endarch. Resin canals are present in the cortex. Medullary rays are broad and the xylem consists of only tracheids.

Companion cells are absent in the phloem Pith is small. Anatomy of old stem exhibits secondary growth, due to which cortex becomes highly reduced and pith gets completely obliterated. Only tracheids are present in the wood. Medullary rays are uniseriate.

Internally; leaf shows xerophytic characters. Single-layered epidermis is thickly-circularized and contains sunken stomata. Hypodermis is single- layered and made of lignified cells. Mesophyll is differentiated into palisade and spongy parenchyma. Resin canals are present in spongy parenchyma region.

A single collateral vascular bundle is present in the centre of the lamina. Phloem faces dorsal side while protoxylem of vascular bundle faces ventral side of the lamina. Biota is monoecious, i.e. both the types of cones develop on the same plant. Male and female cones (Fig. 11.51 B-C), however, develop usually on separate branchlets.

Female cone-bearing branches are usually curved downwards (Fig. 11.51 C). Each female cone (measuring about 20mm in length and 15mm in diameter) contains 3-4 pairs of decussate, fleshy ovuliferous scales, the tip of each of which is spiny and curved.

On the basal part of the ventral surface of each ovuliferous scale are present 1 to 3 ovules (Fig. 11.51 D). Usually the ovule-bearing fertile scales in each cone are only three. Uppermost and a few lower scales are stenle. As many as 10-12 ovules are present in the female cone of Biota. Each ovule (Fig. 11.51 E) has a massive nucellus covered by an integument.

In the upper part, the integument is free from the nucellus and forms a micropyle. Nucellus contains one or two sporogenous cells which divide and form the sporogenous tissue. In the centre of the sporogenous tissue develops a megaspore mother cell which divides reductionally and form a row of three cells.

Lower two cells represent megaspores, of which only the chalazal megaspore remains functional. The uppermost cell of the 3-celled stage is an undivided dyad cell.

Each male cone (measuring about 2.5 to 4mm in diameter) bears 4 to 8 microsporophyll’s arranged in an opposite decussate manner Each microsporophyll (Fig. 11.5IF) is a shortly-stalked body with a slightly broader base and an inwardly curved margin, and contains 3-5 microsporangia on its dorsal surface. Microsporophyll’s get separated exposing the microsporangia in the mature male cones (Fig 11.51 B).

Each microsporangium, when young, is a round or elongated, shortly- stalked body consisting of sprogenous cells surrounded by a three-layered wall (Fig. 11.51 G). The innermost wall layer represents tapetum. The microsporangium development is of eusporangiate type.

The cells of the sporogenous tissue represent microspore mother cells. They undergo reduction division and form tetrads of haploid microspores (Fig. 11 51H). Several starch grains and a large nucleus are present in each microspore. A thin exine and a thick intine surround each microspore (Fig. 11.51I).

Female gametophyte develops from the functional megaspore (Fig. 11.52 A). The nucleus of the megaspore divides repeatedly without any wall formation, and thus results in the formation of several free nuclei (Fig. 11 52B,C) in the young and now enlarged gametophyte. After the formation of about four thousand free nuclei, the wall formation starts in the gametophyte.

Wall formation is centripetal and the entire gametophyte becomes cellular (Fig. 11 52D,E). Mature gametophyte is long and slightly tapered at the chalazal end.

A large number of archegonia (15 to 30 but usually 22) develop near the micropylar end of the female gametophyte in the form of an archegonial complex (Fig. 11.52F). A common jacket layer surrounds the archegonial complex. Several of the cells of this jacket are bi-nucleate.

Male gametophyte develops from the microspore or pollen grain (Fig. 11.51 I). In the initial stages the number of starch grains increases, nucleus moves to one side and divides to form a small antheridial cell and a large tube cell (Fig. 11.52G). Shedding of pollen takes place at this two-celled stage. Its exine bursts as it happens to lie on the nucellus surface.

The intine comes out in the form of a small pollen tube which moves through the nucellar tissue. The antheridial cell becomes free, moves down in the pollen tube and divides to form a stalk nucleus and a body cell (Fig. 11 52H). Sometimes the pollen tube becomes branched. The body cell divides and form two male cells. Stalk nucleus and tube nucleus ultimately dis-organise.

Fertilization process starts after the entry of the pollen tube in the archegonial chamber. Neck cells degenerate and a passage is formed for the entry of the male cells in the archegonium. Fusion between the nucleus of one male cell and the egg nucleus results in the formation of a diploid zygote.

Embryo development starts after the migration of the diploid zygotic nucleus towards the archegonial base (Fig. 11.52 I). This diploid nucleus divides by free-nuclear divisions to form 4 and then 8 nuclei. Soon the wall formation takes place and 8 cells arranged in two tiers are formed.

Lower tier of embryonal cells usually consists of 4 cells but sometimes of 6 or 5 cells also. Upper tier usually consists of 4 cells but sometimes of only 2 or 3 cells.

The cells of upper tier divide to form an open tier and a middle pro-suspensor tier (Fig. 11.52K). Three distinct tiers of cells are thus formed: open tier, pro-suspensor tier and embryonal tier (Fig. 11.52K). The cells of the pro-suspensor tier elongate, thus pushing the embryonal tier into the gametophyte. The cells of the embryonal tier divide and form primary suspensor cells and embryonal cells (Fig. 11 52L).

Primary suspensor cells elongate, and at the tip of each of these cells is present a small embryonal cell. After a period of rest, during which primary suspensor cells elongate too much, each embryonal cell divides by two vertical divisions at right angles to one another. Several irregular divisions in this dividing embryo result in the formation of a globular mass.

Sometimes the secondary suspensors are also formed. All embryos abort gradually except the one which is situated deepest in the gametophytic tissue. A root tip, stem tip and two cotyledons are present in a mature embryo (Fig. 11.52M).

Hypocotyl is formed by the elongation of the region between root tip and stem tip. A mature seed remains surrounded by a 20-25 cells thick seed coat. Seeds show almost no dormancy, and the germination is epigeal.