Phylum coniferophyta (conifers)

• Usually produce cones on whichsporangia, spores and seeds develop
• Seeds are not enclosed in anovary. They lie on the surface of specialised leaves called ovuliferous scalesin structures called cones.
• No fruit because no ovary

Conifers are a successful group of plantsof worldwide distribution, accounting for about one-third of the world’sforests. They are trees or shrubs, mostly evergreen, with needle-like leaves.

Most of the species are found at higheraltitudes and further north than any other trees. Conifers are commerciallyimportant as ‘softwoods’, being used not only for timber but for resins,turpentine and wood pulp.

They include pines, larches (which aredeciduous), firs, spruces and cedars. A typical conifer is Pinus sylvestris, theScots pine.

Pinussylvestris is found throughout central and northernEurope, Russia and North America. It is native to Scotland, though it has beenintroduced elsewhere in Britain. It is planted for timber and ornament, being astately, attractive tree up to 36m in height with a characteristic pink toorange-brown flaking bark. It grows most commonly on sandy or poor mountainsoils and consequently the root system is often shallow and spreading.

Each year a whorl of lateral buds aroundthe stem grows out into a whorl of branches. The roughly conical appearance ofpinus and other conifers is due to the transition from whorls of shorter(younger) branches at the tops to longer (older) branches lower down. Thelatter usually die and drop off as the tree grows, leaving the mature treesbare for some distance up their trunks.

The main branches and trunk continue growthfrom year to year by the activity of an apical bud. They are said to show unlimited growth. They have spirallyarranged scale leaves, in the axils of which are buds that develop into veryshort branches (2-3 mm) called dwarfshoots.  These are shoots of limited growth and at their tips growtwo leaves.

Once the shoot has grown, the scale leaf atits base drops off leaving a scar. The leaves are needle-like, reducing thesurface area available for the loss of water. They are also covered with athick, waxy cuticle and have sunken stomata, further adaptations for conservingwater.

These xeromorphic features ensure that thetree does not lose too much water from its evergreen leaves during cold seasons,when water may be frozen or difficult to absorb from the soil. After two orthree years the dwarf shoots and leaves drop off together, leaving a furtherscar.

Life cycle of a conifer – Pinus sylvestris

The tree is the sporophyte generation. Inspring, male and female cones are produced on the same tree. The male cones areabout 0.5cm in diameter, rounded and found in clusters behind the apical budsat the bases of new shoots. They develop in the axils of scale leaves in theplace of dwarf shoots. Female cones arise in the axils of scale leaves at thetips of new strong shoots, at some distance from the male cones and in a morescattered arrangement.

Since they take three years to completegrowth and development, they are of various sizes, ranging from about 0.5 – 6cm on a given tree.

They are green when young, becoming brownor reddish-brown in their second year.

Both male and female cones consist ofspirally arranged, closely packed sporophylls (modified leaves) around acentral axis.

Each sporophyll of a male cone has two microsporangia or pollen sacs on itslower surface. Inside each pollen sac, meiosis takes place to form haploid pollen grains or microspores. These contain the male gametes. Each grain has twolarge air sacs to aid in wind dispersal.

During May, the cones become yellow inappearance as they release clouds of pollen. At the end of the summer theywither and drop off.

Each sporophyll of a female cone consistsof a lower bract scale and a larger upper ovuliferousscale. On its upper surface are two ovules side by side, inside which thefemale gametes are produced.

Pollination takes place during the firstyear of the cone’s development, but fertilisation does not take place until thepollen tubes grow during the following spring.

The fertilised ovules become winged seeds.They continue to mature during the second year and are dispersed during thethird year. By this time the cone is relatively large and woody and the scalesbend outwards to expose the seeds prior to wind dispersal.