BIOLOGY OF SPERMAPHYTA

Conifers

The well-known Scots pine and the Christmas tree are examples of conifers. These are trees bearing cones. A cone consists of a collection of scales which are borne spirally on a central stalk. Two kinds of cone are formed. One produces pollen and the other ovules. The pollen is carried by the wind and some of the pollen drifts between the scales of the cones bearing ovules into the micropyles of the ovules. Fertilization of an egg cell in each ovule occurs by the fusion of its nucleus with that of a male nucleus carried to it by a pollen tube from a pollen grain germinating inside the micropyle. This process is similar to that in a flowering plant, but since the seeds are naked and not enclosed in an ovary, the pollen can reach the ovules directly and does not have to germinate on a stigma.. After fertilization the cones bearing the ovules enlarge and their scales ultimately become woody. When the fertilized ovules have developed into ripened seeds, the scales of the woody cones which bear them gape and the seeds escape. Each seed has a wing attached to it and is distributed by the wind, its descent being delayed by spinning. The pollen and embryo sacs are the gamctophytes.

Coniferous trees are all evergreens, except the larch. Their leaves are needle-shaped, except in the yew and the cypress. In the pine the leaves are borne in pairs on dwarf shoots which thickly clothe the main stems. Unlike oaks and other forest trees which have broad leaves and are flowering plants, the main stems of the pines, firs and larches grow straight upwards and bear horizontal lateral branches, the lowest ones being the longest, so that the whole tree has a conical outline. In the Scots pine the lower branches die and drop off early, leaving the greater part of the trunk naked. The straight trunks of the conifers are useful for making telegraph and scaffold poles and pit-props.

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