ADVERTISEMENTS:
The following points highlight the eighteen general characteristics of Podocarpaceae.
1. Most of the genera are small or large-sized trees (e.g. Podocarpus) reaching up to 60 metre or more in height but some are small-sized plants attaining a height of only few inches as in some species of Dacrydium.
2. Leaves in most of the members are spirally arranged but in Microcachrys they are arranged decussately.
ADVERTISEMENTS:
3. Leaves way be scale like, needle like, linear or broadly lanceolate.
4. Shoot dimorphism in the form of long and dwarf shoots is absent in most genera except in some species of Phylloclade’s.
5. The male cones are axillary in position and bear spirally arranged microsporophyll’s. Each microsporophyll bears two microsporangia. Sometimes new male cones develop from the base of the older cones.
6. The development of microsporangium in Podocarpaceae is eusporangiate.
ADVERTISEMENTS:
ADVERTISEMENTS:
7. Except Saxegothaea, the pollen grains in Podocarpaceae are winged. Two wings are present in the pollen grains of most of the genera but in Microcachrys and Microstrobos pollen grains have three wings.
8. The female cone in many species is so highly reduced that it is represented only in the form of a swollen fleshy receptacle with a single terminal ovule (e.g. Dacrydium). In some other species, however, the ovules are borne in cones. Generally, the female cones appear unstrobiloid. In Dacrydium franklin and Podocarpus spicatus the female cones look like loose spikes.
9. The ovule is usually reflexed but may also be erect.
10. In all genera of Podocarpaceae, except Microstrobos and Phyllocladus, the ovuliferous scale remains more or less folded round the single ovule in the form of an extra envelope called epimatium. In some species the epimatium remains completely fused with the bract scale while in others it may be partially or completely fused with the integument of the ovule.
11. Usually the megaspore mother cell divides meiotically to form a linear row of three megaspores, of which the uppermost cell remains undivided. However, in Dacrydium and Microcachrys a linear tetrad of four megaspores is formed.
12. The functional megaspore undergoes several free-nuclear divisions. Wall formation in the female gametophyte is centripetal and takes place after some time.
13. Archegonia develop in groups of 20 to 25 in the female gametophyte.
14. In the development of male gametophyte most Podocarpaceae resemble Araucariaceae. More than two prothallial cells develop in almost all cases. The prothallial cells recorded in different genera are 1 to 3 in Phyllocladus, 1 to 8 in Podocarpus, 3 to 4 in Saxegothaea and 3 to 6 in Dacrydium.
ADVERTISEMENTS:
15. Pollination is anemophilous in most of the species.
16. There is a wide range of pollination mechanisms among Podocarpaceae. In Podocarpus the pollen grains have two wings or air bladders. These wings help them to float towards the nucellus having a large amount of exuded fluid. In Saxegothaea, on the other hand, there is no exuded fluid, and pollen grains are also without any air bladders. The pollen grains get themselves alighted on the cone scale.
The nucellus bulges out quite prominently beyond the micropyle of the ovule. The pollen tube grows into the micropyle and thus completes the pollination mechanism. In Saxegothaea the pollen tube remains dormant in these surroundings for a period of as large as nine months.
In the pollen grains of Phyllocladus the wings or air bladders are ill-developed. At the time of pollination the micropyle of the ovules points obliquely upwards. A floatation mechanism of pollination is shown by other members of Podocarpaceae.
ADVERTISEMENTS:
17. Simple polyembryony is observed in Phyllocladus alpinus and in several species of Podocarpus. Several other species also show cleavage polyembryony.
18. Basic chromosome number is highly variable in Podocarpaceae. It ranges from 9 to 19 but in no member 14 and 16 are represented. It varies greatly even within the same genus. The basic chromosome number is 9 in Phyllocladus, 12 in Saxegothaea and highly variable in Dacrydium (9, 10, 11, 12 and 15) and Podocarpus (10, 11, 12, 13, 17, 18 and 19).
