Division Coniferophyta (Conifers).

Division Coniferophyta

Characteristics of Division Coniferophyta

1. They are non-flowering, seed bearing plants, producing naked seeds which are not enclosed in ovaries or fruit tissues.
2. Sexual reproduction involves microspores (male gametophyte) and megaspores (female gametophyte) which are found in male and female cones or strobili respectively.
3. Fertilisation does not require water; instead, they develop pollen tubes which carry sperms to the ovule for fertilisation.
4. They have poor xylem with only tracheids as conducting elements but no vessel elements. This is the reason why most coniferophytes produce soft wood.
5. Their phloem tissues lack companion cells; instead, they are associated with albuminous cells.
6. Leaves are reduced into spiny or needle-like leaves to minimize water loss through transpiration. Exceptions are Gingkos and Cycads.

Structure of Pinus

The sporophyte consists of roots, stem and branches, bearing needle-shaped leaves which appear in clusters of two to five.

These clusters are called fascicles. Their leaves are dark green with cuticle, sunken stomata, and resin canals in the mesophyll.

Their stems develop relatively wider annual rings of the xylem. The constituent tissues of these species have resin canals which run vertically and laterally along the stem. The bark has secondary phloem, which is relatively thick and does not have companion cells, but albuminous cells that perform the same function as the companion cells.

The trunk has a tap root system with lateral roots in different directions. Roots of some Pinus form symbiotic association (mycorrhizae) with fungi. Young shoots have two types of leaves; small scaly leaves on the main stem and leaves on the dwarf shoots. In addition, the plant has foliar spars which develop into normal branches. The fertile plant bears female and male reproductive cones which are distinct when mature. The male cones are relatively soft or herbaceous and small while female cones are woody and relatively larger. Usually, male cones are borne on the lower branches while female cones are borne on the upper branches. The leaves enclosing the cones are called sporophylls. For instance, microsporophylls are associated with male cones, whereas megasporophylls are associated with female cones.

Adaptations of Pinus to its Mode of Life

1. The sporophyte is differentiated into roots which are well developed and grow deep into the soil for anchorage and absorption of underground water.
2. The stem is reddish brown in colour and covered by scaly leaves to prevent water loss through transpiration.
3. The leaves are needle like to minimize loss of water (as surface area is reduced).
4. The leaves are evergreen for photosynthesis.
5. The stomata are sunken which reduce water loss through transpiration.
6. The leaves have hypodermis below the epidermis which prevent further desiccation.
7. They have the conducting tissue, the xylem and phloem, for upward movement of water and mineral salts (xylem) and transport of food (phloem).
8. They have developed mechanical tissues (woody stem) i.e. sclerenchyma and collenchyma which provide mechanical support as the plant has to grow into tall trees.
9. The microspores and megaspores have scales which prevent them from water loss.
10. The presence of air sacs in the microspores facilitates wind dispersal thus conifers do not need water for fertilisation since male gametes are blown by wind to reach the female gametes.
11. The microspores develop a resistant wall for protection and they are small and light to be blown by wind.
12. They produce a large number of seeds in the cones to increase chance of survival.
13. Formation of resin canals that resist entry of fungi and other pathogens when the plant is injured. The resin normally covers up the injured part and the seed bearing habit enables the plant to survive during adverse conditions (unfavourable conditions) as the seeds are protected by the seed coat.
14. The stored food in the seed can be used by developing embryo during germination.