In this article we will discuss about the aspects of the life cycle of Podocarpus.
Podocarpus, represented by over 110 species, is not only the largest genus of Podocarpaceae, it is also the largest genus of all present-day conifers. It is widely represented in western hemisphere extending from Patagonia to West Indies, and in eastern hemisphere extending from New Zealand to Japan and Myanmar.
Majority of the species are tall trees reading up to 40 to 60 metres or more in height. Several species are shrubs and Podocarpus ustus is a partial parasite on the roots of Dacrydium toxoids. It is perhaps the only parasitic conifer known.
Leaves in most of the species are spirally arranged but in some they are also opposite decussate. They reach up to 30cm in length and 5cm in breadth in some species while in some other species they are scale-like and minute.
The male cones (Fig. 11 53A,C) have several spirally arranged microsporophyll’s. In Podocarpus macrophylla the cones are quite long bodies. Each microsporophyll possesses two microsporangia. In the microsporangia of mature cones are present a large number of pollen grains. Each pollen grain has two wings. Microsporangium development is of eusporangiate type.
The sporangial wall is 5-7 layered, of which the innermost layer functions as a tapetum. Each pollen grain develops into a male gametophyte. Male pro-thallus cells are 1-8 in number in different species of Podocarpus. The spermatogenous cell or body cell divides into three unequal male gametes. According to Konar and Oberoi (1969) there are only two male gametes in P. gracilior.
The female cone is generally unstrobiloid i.e. it is not a compact cone-like body. It is a uniovulate or bi-ovulate structure situated at the top of a flattened short segment in Podocarpus macrophylla. The ovule-bearing segment also contains a few reduced leaves. The ovule remains enclosed completely or partially by the folded ovuliferous scale, which represents epimatium.
In P. spicatus the female cones appear like loose spikes, and the epimatium remains fused with the integument throughout its length. In P. ferruginens, however, the integument remains fused with the epimatium only at its chalazal end. The ovule is erect and anatropous (Fig. 11.53D).
In P. gracilior the integument is slightly larger than the epimatium. The female gametophyte remains enclosed by the nucellus. The endosperm and the archegonial complex are present in the female gametophyte. The megaspore mother cell in P. gracilior and some other species lies quite deep in the nucellus tissue. It divides meiotically and form a linear tetrad of four megaspores in P. gracilior and P. falcatus.
After the formation of several free nuclei in the megaspore and prior to the start of wall formation, the female gametophyte passes through a long resting period. Wall formation is centripetal. The archegonia develop in groups of 15 to 25 in different species.
They are oval with round base in P. gracilior (Fig. 11.53E) and long with pointed base in P. andinus. Archegonial neck consists of 4-6 cells arranged in a single tier in P. gracilior.
Pollination is anemophillous in most of the species. In P.gracilior the pollen tube is extensively branched and enters the archegonium laterally. Fusion between the male and female nuclei occurs in the upper part of the archegonium
In Podocarpus amarus, P.andinus, P gracilior etc., the zygotic nucleus divides and re-divides five times to form as many as 32 free nuclei. Some of these free nuclei in P.gracilior move towards the upper part of the archegonium. They persist for sometime or degenerate and are called relict nuclei. Wall formation soon starts forming a lower tier of 9 to 12 embryonal cells and upper open tier of remaining cells.
The cells of the upper open tier divide and form a middle suspensor tier and an open upper tier (Fig. 11.53F). Several cells of the embryonal tier are bi-nucleate. Long suspensor cells push the embryonal cells into the gametophyte.
Embryo tetrad stage is resulted due to the nuclear division and wall formation in the bi-nucleate embryonal cells. P.gracilior shows cleavage polyembryony while several other Podocarpus species (e.g., P. spicatus) show simple polyembryony.