| Pinophyta: conifers | ||||||
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Pinophyta is one of 13 or 14 division level taxa within the Kingdom Plantae. The Division Pinophyta includes all of the conifers (and in some classification schemes the ginkgo tree). In a broader sense, the Pinophyta would be equivalent to the gymnosperms, although such a grouping is clearly polyphyletic. In the more modern, narrower sense it is equivalent to the former Coniferales. A pinophyte is a cone-bearing, seed plant with vascular tissue, typified by trees such as fir and pine or shrubs such as some junipers. The division contains but one class (Class Pinopsida) of living plants. The Class Pinopsida is split into two orders: Order Pinales and Order Taxales
The world's tallest, heaviest, and thickest trees are all pinophytes, and all in the Family Taxodiaceae. The tallest is Coast redwood, with a height of 111 metres (364 feet). The heaviest is Giant sequoia. The thickest, or tree with the greatest trunk diameter, is Montezuma bald cypress.
The leaves of most pinophytes are long thin needles, and the plants have a distinctly scented resinous sap. A few pinophytes, such as the cypress, monkey puzzle tree, and juniper have scale-like leaves instead of needles. The scale-leaves can, like the needle-leaves, be hard and spiky. As an adaptation to cold, the needles contain little sap. The stomata are in long, thin ridges along the needle and can be closed when it is very dry or cold. The leaves are usually dark in colour and thus absorb a maximum of heat from the feeble sun at high latitudes. They are also very acidic and usually remain on the plant for about seven years at a time. Because most species are not deciduous, pinophytes are often called evergreens.
Giant sequoia trees keep their needles for up to forty years. The larch (Genus Larix) and some others are deciduous pinophytes lose their needles every autumn. This may be because they grow where it is very dry. No loss of moisture through stomata can be risked. The Bald cypress is also winter deciduous or semi-deciduous, although a tree of swamplands.
Pinophytes are sometimes said to have an "iron grip" on the land where they grow. There are two reasons for this: One, conifer seedlings have small tufts of needles and thick bark. These provide resistance to fire. The tufts burn quickly and at a very low temperature. The seedlings of other trees lack these protections. Thus, when the fire moves on, the other trees have been cooked but the young conifers are still alive. The second reason is the fact that fallen needles decay slowly creating a dense mat on the forest floor. As they decay, they contribute acidity to the soil. This tends to make the soil unsuitable for most plants.
Pinophyte seeds develop inside woody protective cones loosely called "pinecones" (which technically come only on pine trees). These cones take up to a year to develop to maturity. As they mature they split open allowing the seeds to fall out. Ripe cones remain on the plant for a varied amount of time before falling to the ground.
Male cones have structures called sporangia which produce yellowish pollen. Pollen is released and carried by the wind to female cones. These cones excrete a sticky substance. When a pollen grain lands near a female gametophyte, it undergoes mitosis and fertilizes the female gametophyte. The resulting zygote develops into a seed which can take several years to mature. Eventiually the seed falls to the ground and, if conditions permit, grows into a new tree. Some pinophytes have male and female cones on the same plant (monoecious species); others have them on different plants (dioecious species).
Life cycle