Short notes on the General Structure and Reproduction of Cycas and Pinus
The plant belongs to the sprophytic generation and is differentiated into root, stem and leaves. The adult cycas plant is about 2 meters high. C. media is the tallest species. The stem is thick, unbranched (caudex, but branches may arise when terminal bud is injured) and bears triangular leaf bases on its surface. The leaves show circinate vernation and are loosely arranged. Cycas leaves are of two types (a) scale leaves (b) green foliage leaves. The number of foliage leaves formed in one year will be constant for any given species.
The petiole is stout and called rachis. Rachis bears leaf lets or pinnules on either side. The lower leaf-lets of rachis are modified into spine. Lateral veins are absent in pinnule. The primary root persists and forms a tap root system. The lateral branches of the root come out of the soil (apo-geotropic) and branch repeatedly to form dwarf dichotomously branched coral like man called coralloid roots or corallrhiza. Blue green algae Anabaena live symbiotically in the coralloid roots. It fixes the nitrogen.
The vascular bundles in stem are conjoint, collateral open and arranged in a ring. The xylem of the bundles consists of tracheids only. The pholem comprises of sieve tube and phloem parechyma, they lack companion cells. The secondary growth in a young stem is normal, but as it becomes older, new rings are formed giving rise concentric rings of secondary vascular tissues (polyxylic condition).
Transfusion tissue is present in the leaves which help in conduction. Tough and leathery texture and other structures such as strongly cutinized thickend hypodermis, sunken stomata restricted only to the lower surface, presence of transfusion tissue all these point to a xerophytic adaption of cycas.
Internal Structure of Pinnule:
Show xerophytic nature. Mesophyll is differentiated into palisade and spongy tissue. Vascular bundle of innule is conjoint, collateral, xylem is diploxylic, the larger patch is the centripetal xylem and the two lateral patches are centrifugal xylem. Phloem is present below the centrifugal xylem.
Anatomy of Root:
Resembles that of a dicot root. Xylem is diarch or triarch. In the coralloid root, the algal zone is in the cortex, which is termed as middle cortex.
Cycas is dioecious. The male plant has a cone at its apex. Each cone has a thick central axis around which microsporophylls are spirally arranged. Groups of sporangia or sori are present on the abaxial (lower) side of the sporophyll.
Each sorus contains 2-6 sporangia or anthers which are filled up with pollen grains or microspores. Microspore represents the first cell of male gametophyte stage. The upper part of microsporophyll is sterile and pointed called apophysis.
There is no true female cone is cycas. The megasporophyll arise at the apex of the female plant and are loosely arranged in rosette manner. On either side of the broader part of sporophyll 4-6 red coloured ovules or megasporangia are arranged. Cycas ovule has a single, thick three layered integument which is hard and inner layers are fleshy and the middle layer is hard and stony.
Deep in the nucellus of the mature ovules lies the megaspore mother cell which divides into four megaspores. Only one of the four megaspores is functional and rest three degenerate. By repeated divisions of the functional megaspore, female gametophyte is formed. On the upper surface of the female gametophyte. 2-8 archegonia are formed, each having a ventral cell and an egg.
The pollen grains reach the ovule through the medium of air. The method of fertilization in said to be siphano-zooidiogamy (because of multiciliate antherozoids and tube). In Cycas polyembryony is often seen as egg of almost all the archegonia are fertilized and produce embryo. In Cycas, alternation of generation in quite prominent.
Bulbils (resting adventitious buds) are produced on the stem in the axil of scale leaves. They break up from the parent plant and germinate to give rise to new plant.
The Pinus tree represents the sporophytic generation. The stem displays the excurrent habitat. The main stem is branched. Branches are of two types:
(a) Long branches:
Branches with unlimited growth (grow by means of apical bud)
(b) Dwarf branches:
Branches with limited growth, arising directly from the trunk.
Leaves are also of two kinds:
(a) Foliage leaves:
These are unusual type being long, narrow, tough, green and are frequently known as Pinus-spur or Pine-needles. They are borne only on the dwarf shoots in clusters of two (P. merkusii), three (P. roxburghii) or five (P. wallichana).
Branches with foliage leaves are called spurs. Spurs could be monofoliar, bifoliar, trifoliar, pentafoliar having 1, 2, 3, 5 foliar leaves.
(b) Scale leaves:
These are brown, membranous and are protective in function. These are borne on both type of branches, but they fall off as the dwarf shoots mature. Scale leaves of dwarf shoots are called cataphylls.
Primary root persists and forms a typical elongated straight tap root. Mycorrhizal roots develop like that of cycas.
It is composed of small pith, a thick vascular cyclinder made up of a ring of separate collateral and open vascular bundles. Well developed resin canals are present in the stem. Primary xylem contains neither true vessels (tracheae) nor wood fibres which characteristic of angiosperms. Mesophyll cells not differentiated into palisade and spongy tissue in the leaves. Resin canals and transfusion tissue are present in the leaves.
Anatomy of Root: Resembles dicots. There are 2, 3 or 4 (diarch, triarch or tetrach) exarch bundles (more or less Y shaped) with alternating xylem and phloem (radical). Pith is generally absent. In the apical root meristem there is no dermatogen.
Pinus is monoecious, it bears male and female reproductive cones on the same tree but on separate branches.
It is shortly stalked and consists of an elongated central axis, bearing a number of small spirally arranged and closely fitted scale-like microsporophylls. Numerous winged microspores are produced from microspore mother cell in the microsporangium.
The microspore nucleus divides into a small protallus cell and a large central cell. The nucleus of large central canal cell called the antheridial cell divides into a generative cell and a tube cell.
The tube cell grows out to form a delicate pollen tube which grows into the nucellar tissue upon which it now depends for its nourishment and protection. The pollen tube rests for about a year in this condition because the ovule is not yet ready for fertilization. Hence further growth of microgametophyte is arrested. It rests throughout the late summer and following winter resuming activity in the following April (second year).
The tube becomes active again and it penetrates the nucellar tissue. The generative cell divides to give rise to a barren stalk cell (sterile cell) and a fertile body cell (Spermatogenous cell). The body cell along with protoplasmic contents of the tube and stalk cell pass down the pollen tube. The body cell divides into two unequal cells, which are the male gametes. The gametes are formed only a week before the fertilization.
Arise singly or in a small cluster of two to four, each as a bud in the axial of the scale leaf towards the end of the new shoots of unlimited growth which do not bear the male cones. The female cones are very slow in growth. They take almost a year to be mature enough to receive the pollens. The central axis bears paired scales in a close spiral.
Bract scales or Carpellary scales (Each corresponding to a carpel or megasporophyll), lower scale, small, leathery, brownish scales.
It bears two sessile ovules on its upper surface at the base. Each ovule is orthotropous, and consists of a central mass of tissue the nucellus, surrounded by single integuments made of three layers.
Takes place in March/April in Eastern Himalayas. The amount of pollens liberated by the pine forests at this time is prolific so that the air gets saturated with them and there is a yellow deposit of pollens on the forest floor. This phenomenon is known as ‘sulphur-shower’.
As the ovule matures for the pollination, the nucellar cells dissolve just below the micropyle. The dissolved tissue becomes mucilagenous and project out through the micropyle in the form of a droplet and is called pollination drop.
The pollen tube on reaching the archegonial neck (which takes place a year after pollination i.e. two years after the female cone first emerged) the pollen tube destroys the neck cell. Just before fertilization, the body cell divides into two nake male cells or gametes.
(A number of embroys from a number of fertilized eggs) is common is some genera of Pinaceae viz. Larix & Picea but is Pinus (also Cedrus) the proembryo tires split from one another-into 4-along the 4-cells of each tire giving rise to 4-separate embryos. This is known as cleavage polymebryony. Usually one of these embryos survive in the seed.