Podocarpaceae

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Podocarpaceae

Description

Monoecious or dioecious trees and shrubs, some prostrate (and one parasitic on another member of the family, Parasitaxus, in New Caledonia). Leaves of many shapes and sizes. In one genus there is no scale while this and one other genus have erect ovules, but in the great majority of genera the ovule is inverted. This can yield plum-like fruits or, in other cases, structures resembling those of the cashew (Anacardium occidentale). The basic seed producing structure in Podocarpaceae is a compound terminal or lateral cone in which fertile scales arise in the axils of cone bracts.

Distribution

Antarctic: present Asia-Temperate: Asia-Tropical: New Caledonia: present New Zealand: present tropical African highlands: present tropical American highlands: present
There is a strong Antarctic relationship with a broad extension into Malesia for the 172 known species in 13 genera (in Malesia 1 genera with 61 spp.). Of six local and generally primitive genera, four are in the Antarctic zone and two are in New Caledonia. Three wide-ranging genera extend also into the tropical American highlands and two of these further range across the tropical African highlands. All seven of the wide-ranging genera are common in Malesia, only one of which (Falcatifolium), however, is confined to the Asian tropics; five are in New Zealand. A few species reach into moist subtropical forests of eastern Asia.

Ecology

Mostly trees of moist forests at all elevations and well into the middle latitudes both as major canopy trees and as understory plants, rarely in areas with a marked dry season. A few spècies are scrubby or even prostrate and as such may be found beyond the tree line, on rocky outcrops or other specialized habitats. The family is well represented in, but not confined to difficult soils such as sand and ultrabasics as well as in mossy forests.

There are nodules regularly present on the roots, but their function is unclear. FURMAN () showed that they contained endotrophic mycorrhizae and that nitrogen fixation did not occur. BECKING () indicated that the mycorrhizae were Phycomycetes. Growth is possible in sterile soil without mycorrhizae.

Most genera are dioecious and pollination is by wind. Individuals are usually scattered but locally common and the large quantities of pollen that are often produced seem to be able to reach effectively across considerable distances. Seedlings are found scattered and even quite isolated from seed sources due no doubt to dispersal by birds or fruit bats which eat the fleshy fruit. Coordination of fruiting times is for many species in tropical regions not well developed because at any given time it is often possible to find examples at any and all stages of reproduction and the collection of ripe fruit is variously reported for a given species at disparate dates. I have even seen two stages on the same tree.

Growth is distinctly cyclic and in some genera there are elaborate terminal buds and similar buds for pollen cones. Seed-bearing structures are usually produced on the latest shoots while pollen cones frequently emerge from shoots of the previous cycle. Sometimes leaves of only the last cycle persist on a tree but more common is the display of three or four cycles of growth.

Seeds germinate on or near the surface of the forest floor. The cotyledons remain at least partly inside the seed coat absorbing nourishment from the endosperm while the radicle penetrates the soil and begins forming a root system. Eventually the linear cotyledons shed the emptied seed coat and persist at the base of the growing shoot for a variable length of time. When functioning leaves are established, the cotyledons will be shed. Even if the adult leaves have some other form, the first foliage leaves in almost all taxa are bifacially flattened, often with an abrupt transition where the adult foliage is distinct.

Various parasites are known for this family. Members of Podocarpaceae are the exclusive hosts of three genera of fungus in the family Coryneliales, in Malesia recorded for Podocarpus crassi-gemmis. Their fruiting bodies can often be seen erupting from leaves or stems in Podocarpus or Nageia, but this does not seem to be particularly harmful. One species of Korthalsella (Viscaceae), a dwarf mistletoe, is also parasitic in Podocarpaceae: K. dacrydii has been reported both on Dacrycarpus and on Dacrydium in various parts of Malesia ().

Taxonomy

Two recent works have treated all of what is recognized as a single family here. GAUSSEN (Les Gymnosperms actuelles et Fossiles, fasc. 13 & 14, 1974 & 1976) separates each of the three most distinct genera into families of their own, viz. Saxegothaeaceae, Phyllocladaceae, and Pherosphaeraceae. He recognizes one section of Nageia (Afrocarpus) as a distinct genus, while grouping the rest of this genus and Parasitaxus with Podocarpus. There are eight genera in Podocarpaceae as he envisions it. In my taxonomic revision () I recognize a single family and 13 genera (including the recently published Halocarpus by implication only).

Cytology

According to HAIR & BEUZENBERG () the chromosomes in Podocarpaceae are remarkable. Basically the number for the great majority is in effect n = 10 while for Phyllocladus it is n = 9 (and for extra-MaX. Halocarpus n = 8). For a great many species in most genera, however, there are two kinds of chromosomes. One type, always present, is median to submedian, while the other, sometimes present, is subterminal to subtelocentric. Two of the latter always correspond to one of the former indicating either a progressive splitting of some of the chromosomes or less likely a progressive pairwise fusion of some or all of the chromosomes. Phyllocladus and the genera with bilaterally flattened leaves (Dacrycarpus, Falcatifolium, and Acmopyle) have only the one kind of chromosome. The large genera Dacrydium, Nageia, and Podocarpus are partly with one kind and partly mixed. The other six (mostly small) genera always have mixed chromosome types. The result is a wide range of actual chromosome numbers from n = 8 to n=19.

Uses

The wood of trees in this family is light coloured, usually yellowish, is durable, easy to work, and generally similar to pine though rather harder. It is extensively used for lumber where sufficiently dense stands of good-sized trees occur, mostly outside of Malesia. In Borneo wood of Nageia is sometimes mixed with Agathis ('dammar') in commercial cuttings. Specimens of many genera are selected for planting around native settlements although the specimens seen in urban areas within Malesia usually come from China or Japan. In fact, natives in many areas so prize the wood for construction that, as I have been told on several occasions and have confirmed through experience, it is often necessary to go some distance from the nearest village to find mature wild trees. In some species the fruits are edible and I have found a few in tropical America that were locally appreciated but I have not discovered any such example in Malesia.

Notes

Conifers lack flowers and even where brightly coloured fruit occurs it tends to be very transitory, thus conifers tend to be bypassed by collectors. Most genera are dioecious and separate collections of male and female are necessary. It is often desirable to have a juvenile specimen (low branches in the shade usually have the juvenile form) to appreciate the range of foliage form. Sometimes immense numbers of recently shed pollen cones are encountered on the forest floor and these are worth collecting.

Embryology

The fertilized egg undergoes four or five mitoses resulting in up to 16 to 32 free nuclei. Most of these are then walled off and cluster at the base of the archaegonium forming a pro-embryo of several tiers of cells. Those in the lowest tier are embryonic and divide to form binucleate cells of which there may be but one to in some genera as many as 16. The next tier of cells elongates into a 'prosuspensor' consisting of from 3 to 25 cells, the number of cells being roughly proportional to the size of the seed and therefore the length needed to reach the centre of the female gametophyte (later to become endosperm). A third tier of cells is not completely walled off and is left behind to degenerate as the embryonic mass is projected away. At the apex of the embryo there may be one or a few cells forming a 'cap'. Unless there are five mitoses (i.e. Nageia and Prumnopitys — both with large seeds) a larger number of suspensor cells means fewer embryonic cells. In the majority of cases the embryonic mass divides, along with the secondary suspensor which it generates, into several competing units, the common conifer condition known as cleavage polyembryony. Simple polyembryony resulting from more than one fertilized archae-gonium also occurs. Growth of the embryo begins when the nuclei of the binucleate cells divide and then form groups of four cells. Probably an actual developed embryo derives from but a single binucleate cell so that, when there are more, they are competitive. The reduction of the number of embryonic cells often to a single cell in the genus Podocarpus appears to be a derived character. The binucleate embryo stage itself is unique in Podocarpaceae, while the number of mitoses leading to the pro-embryo is intermediate between a large number for Araucariaceae and non-coniferous Gymnosperms on the one hand and a smaller number for most other conifers on the other. Sciadopitys in the Taxodiaceae has five, while Cephalotaxus and most of the Taxaceae also have four.