- Usually produce cones on which sporangia, spores and seeds develop
- Seeds are not enclosed in an ovary. They lie on the surface of specialised leaves called ovuliferous scales in structures called cones.
- No fruit because no ovary
Conifers are a successful group of plants of worldwide distribution, accounting for about one-third of the world’s forests. They are trees or shrubs, mostly evergreen, with needle-like leaves.
Most of the species are found at higher altitudes and further north than any other trees. Conifers are commercially important as ‘softwoods’, being used not only for timber but for resins, turpentine and wood pulp.
They include pines, larches (which are deciduous), firs, spruces and cedars. A typical conifer is Pinus sylvestris, the Scots pine.
Pinus sylvestris is found throughout central and northern Europe, Russia and North America. It is native to Scotland, though it has been introduced elsewhere in Britain. It is planted for timber and ornament, being a stately, attractive tree up to 36m in height with a characteristic pink to orange-brown flaking bark. It grows most commonly on sandy or poor mountain soils and consequently the root system is often shallow and spreading.
Each year a whorl of lateral buds around the stem grows out into a whorl of branches. The roughly conical appearance of pinus and other conifers is due to the transition from whorls of shorter (younger) branches at the tops to longer (older) branches lower down. The latter usually die and drop off as the tree grows, leaving the mature trees bare for some distance up their trunks.
The main branches and trunk continue growth from year to year by the activity of an apical bud. They are said to show unlimited growth. They have spirally arranged scale leaves, in the axils of which are buds that develop into very short branches (2-3 mm) called dwarf shoots. These are shoots of limited growth and at their tips grow two leaves.
Once the shoot has grown, the scale leaf at its base drops off leaving a scar. The leaves are needle-like, reducing the surface area available for the loss of water. They are also covered with a thick, waxy cuticle and have sunken stomata, further adaptations for conserving water.
These xeromorphic features ensure that the tree 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 or three years the dwarf shoots and leaves drop off together, leaving a further scar.
Life cycle of a conifer – Pinus sylvestris
The tree is the sporophyte generation. In spring, male and female cones are produced on the same tree. The male cones are about 0.5cm in diameter, rounded and found in clusters behind the apical buds at the bases of new shoots. They develop in the axils of scale leaves in the place of dwarf shoots. Female cones arise in the axils of scale leaves at the tips of new strong shoots, at some distance from the male cones and in a more scattered arrangement.
Since they take three years to complete growth and development, they are of various sizes, ranging from about 0.5 – 6 cm on a given tree.
They are green when young, becoming brown or reddish-brown in their second year.
Both male and female cones consist of spirally arranged, closely packed sporophylls (modified leaves) around a central axis.
Each sporophyll of a male cone has two microsporangia or pollen sacs on its lower surface. Inside each pollen sac, meiosis takes place to form haploid pollen grains or microspores. These contain the male gametes. Each grain has two large air sacs to aid in wind dispersal.
During May, the cones become yellow in appearance as they release clouds of pollen. At the end of the summer they wither and drop off.
Each sporophyll of a female cone consists of a lower bract scale and a larger upper ovuliferous scale. On its upper surface are two ovules side by side, inside which the female gametes are produced.
Pollination takes place during the first year of the cone’s development, but fertilisation does not take place until the pollen tubes grow during the following spring.
The fertilised ovules become winged seeds. They continue to mature during the second year and are dispersed during the third year. By this time the cone is relatively large and woody and the scales bend outwards to expose the seeds prior to wind dispersal.