Pinus

Pinus (Class Coniferopsida, Order Coniferales) is a tall, conical, evergreen tree forming dense forest across the North temperate and sub-alpine world, usually at 1,200–3,000 m altitude; species include P. roxburghii, P. wallichiana, P. gerardiana and P. insularis.

The sporophyte is differentiated into root, stem and leaves. The root is a tap-root system with poorly developed root hairs, instead relying on fungal mycorrhizae. The stem is cylindrical, erect, woody and branched, with monopodial branching of two distinct types: long shoots (branches of unlimited growth, on the main trunk, shortening gradually toward the tip to give the whole tree its pyramidal shape, and bearing only scale leaves) and dwarf shoots (branches of limited growth, lacking apical buds, arising in the axils of scale leaves and bearing both scale and foliage leaves). There are correspondingly two leaf types: small, brown, membranous scale leaves (called 'cataphylls' on the dwarf shoot, protecting young buds) and green, needle-like foliage leaves borne in a cluster (a 'foliar spur') on each dwarf shoot — the number of needles per spur is diagnostic: one in P. monophylla, two in P. sylvestris, three in P. gerardiana, four in P. quadrifolia and five in P. excelsa.

Internally, the root shows an epiblema, a wide resin-duct-bearing parenchymatous cortex, a suberised, tannin-impregnated endodermis, a multilayered pericycle, and a radial diarch stele with exarch xylem forming a Y-shape around a resin duct, plus secondary growth. The stem shows a heavily cutinised epidermis, sclerenchymatous hypodermis, a resin- and tannin-rich parenchymatous cortex, an indistinct endodermis, a ring of five or six conjoint, collateral, open, endarch vascular bundles, a pith with medullary rays, and clear annual growth rings from secondary growth. The needle leaf shows marked xerophytic adaptation: a roughly triangular cross-section, thick cuticle, sunken stomata opening into sub-stomatal cavities, sclerenchymatous hypodermis, undifferentiated but deeply infolded mesophyll (the infoldings boost photosynthetic surface area), a resin canal, an endodermis, a starch-storing pericycle, and two conjoint, collateral, open vascular bundles separated by sclerenchyma and served by specialised albuminous cells (mesophyll-to-phloem transport) and tracheidal cells (transfusion tissue, for water conduction).

Pinus is heterosporous and monoecious, with separate male and female cones on different branches of the same plant. Male cones form in clusters on branches of unlimited growth, each a central axis ringed by spirally arranged microsporophylls bearing two abaxial microsporangia packed with numerous winged microspores (pollen — the male gametophyte). Female cones form in groups of 1–4 in the axils of scale leaves and take about three years to mature; each is built around a central axis bearing spirally arranged megasporophylls, each megasporophyll a compound structure of a sterile bract scale and a fertile ovuliferous scale carrying two ovules (bearing megaspores, the female gametophyte) on its dorsal surface.

Pollination is anemophilous; pollen is shed at the four-celled stage (two prothallial cells, one generative cell, one tube cell), and a sticky secretion from the ovule's micropyle traps pollen grains and draws them into the pollen chamber. The tube cell grows out as a pollen tube, while the generative cell divides into a stalk cell and a body cell, which itself divides into two unequal male cells. Fertilization happens roughly a year after pollination, when the pollen tube (carrying two male nuclei) reaches the egg through the micropyle; one male nucleus fuses with the egg to give a diploid zygote (the other degenerates). The zygote develops mitotically into an embryo (polyembryony is common), which matures into a winged seed; germination is epigeal. As in every gymnosperm covered in this chapter, the life cycle shows a clear alternation of generation. …