Pinus - Gymnosperms

Pinus

Class – Coniferopsida

Order – Coniferales

Family –Pinaceae

Genus - Pinus

Pinus is a tall tree, looks conical in appearance and forms dense evergreen forest in the North temperate and sub-alpine regions of the world. They mostly grow in high altitudes (ranging from 1,200 to 3,000 metres). Some species of this genus include, Pinus roxburghii, P. wallichiana, P. gerardiana and P. insularis.

External features

The plant body is sporophyte and is differentiated into root, stem and leaves.

The main stem is branched. The branches are dimorphic with long and short branches (Figure 2.48).

Root

Tap root system is found in Pinus. The root hairs are not well developed and the roots are covered with fungal hyphae called mycorrhizae.

Stem

The stem is cylindrical, erect, woody and branched. The branches are monopodial. The branches are of two types.

(i) Long shoots or branches of unlimited growth, (ii) Dwarf shoot or branches of limited growth

(i)       Long shoots or branches of unlimited growth

The long shoot is present on the main trunk the apical buds grow indefinitely, They shorten gradually towards the tip, thus providing a pyramidal appearance to the tree. These branches bear scale leaves only.

(ii) Dwarf shoot or branches of limited growth

These branches do not have apical buds and hence show only limited growth. They develop in the axils of scale leaves and bear both scale and foliage leaves.

Leaves

There are two types of leaves 1. scale leaves, 2. foliage leaves

1. Scale leaves:

They are dark, brown, membranous, thin and small. They are present on both long and dwarf shoots. Their function is to protect young buds. The scale leaves on the dwarf shoots have a distinct midrib and are called “Cataphylls”.

2. Foliage leaves:

The foliage leaves are green angular and needle like structures. They are borne on the dwarf shoot. A dwarf shoot with a group of needle like foliage leaves is known as foliar spur. The number of needles per dwarf shoot varies among the species. It may be one (Pinus monophylla), two (P. sylvestris), three (P. geraradiana), four (P. quadrifolia) and five (P. excelsa).

Internal Structure

T.S. of root

The internal structure of root reveals the presence of epiblema, cortex and stele.

The epiblema is made up of single layer of parenchymatous cells. Cortex is the wide zone and consists of parenchyma. Some of the cells have resin ducts. A single layered endodermis with suberised wall is present and is impregnated with tannins.A multilayered pericycle made up of parenchyma is present. Vascular tissue is radial, diarch with exarch xylem. The protoxylem bifurcates to form a ‘Y’ shaped structure and a resin duct lies in between the two arms of protoxylem. Secondary growth is present (Figure 2.49).

T.S. of Stem

The internal organization of the stem shows three regions namely epidermis, Cortex and vascular tissue (Figure 2.50).

Epidermis is the outermost layer composed of compactly arranged and heavily cutinized cells. Epidermis is followed by few layers of sclerenchymatous hypodermis. The cortex consists of thin walled parenchyma cells. Resin canals and tannin filled cells are present in this region. Endodermis is indistinguishable from cortical cells. Vascular region is surrounded by pericycle. A ring consists of five or six vascular bundles are present. Vascular bundles are conjoint, collateral, open and endarch. Pith and medullary rays are present. Secondary growth is present and annual rings are formed.

T.S. of needle or foliage leaf

The internal structure of needle shows xerophytic adaptations. In cross section the outline appears more or less triangular and is divided into epidermis, mesophyll and vascular bundles. The epidermis is single layered and possesses thick cuticle and sunken stomata.Epidermis is followed by a few layers of sclerenchymatous hypodermis. It is interrupted by sub-stomatal cavities (Figure 2.51).

Mesophyll is not differentiated into palisade and spongy parenchyma. Thin walled cells with chloroplasts are present. The cells are peculiar with numerous small infoldings which project into the cavities. The infoldings increase the photosynthetic area of the needle leaves Resin canal is present in the mesophyll. A single layered endodermis separates the vascular region from the cortex. A multilayered pericycle containing starch is present. Two types of specialised cells called albuminous cells and tracheidal cells are present. The former helps to pass substances from the mesophyll to the phloem while the latter helps in water conduction and constitutes transfusion tissue. Two vascular bundles are present. They are separated by sclerenchyma tissue. The Vascular bundles are conjoint, collateral and open.

Reproduction

Pinus is heterosporous and produces two types of spores called. microspores and megaspores. The plants are monoecious. Both male and female cones or strobili develop on the different branches of the same plant (Figure 2.52). 

Male cone

Male cones are produced in clusters on branches of unlimited growth. Each cone develops on the axil of scale leaf . The male cone consists of a centrally located cone axis surrounded by numerous spirally arranged microsporophyll. It bears two microsporangia at the base of the abaxial side of the microsporophyll. Each sporangium bears numerous winged microspores (n) or pollen grains. The microspores represent the male gametophyte.

Female cone:- 

Female cones are formed in the groups of 1 to 4 in the axils of the scale leaves. The female cone takes about three years to mature. It has the central axis around which megasporophylls are arranged spirally. The megasporophyll is the compound structure consisting of two types of scales. 1. Bract scale (sterile), and 2. Ovuliferous scales (fertile). The dorsal surface of each ovuliferous scale bears two ovules. Ovules bear megaspores which represent the female gametophyte. 

Pollination and fertilization

In Pinus wind pollination takes place (Anemophilous). The microspore or pollen grain is released in the 4 celled stage(two prothallial cell, 1 generative and 1 tube cell). At the time of pollination a secretion oozes out from the micropyle of the ovule which entangles pollen grains which helps to lodge them in the pollen chamber. The tube cell protrudes to form pollen tube. The generative cell divides to produce stalk cell and body cell. The body cell divides into unequal male cells. Fertilization takes place after about a year of pollination. The pollen tube containing two male nuclei penetrates through the micropyle and reaches the egg. One of the male nuclei fuses with the egg forming diploid zygote and the remaning one gets degenerated. The fertilized egg (zygote) undergoes mitotic division and develops into an embryo.

Polyembryony is present. The embryo undergoes several changes and finally becomes a winged seed. The seed germination is epigeal. Life cycle of Pinus shows alternation of generation (Figure 2.53).