Pinus : Classification Anatomy Morphology and Reprodction |Notes. The conifers constitute more than three- forth of the living gymnosperms flora, and cover vast area of the temprate region of both northen and southern hemisphere. Coniferales is hte lasgest order of gymnosperms with about 52 genera and 550 species.
Division – Coniferophyta
Pinus, one of the most important Coniferales, is represented by about 105 species. These are mainly distributed in the Northern Hemisphere, and found commonly in Northern Europe, Northern and Central America, subtropics of North Africa, India, Myanmar, Pakistan, Afghanistan, Indonesia, etc.
Six species of Pinus (P. roxburghii, P. wallichiana, P. insularis, P. gerardiana, P. armandii and P. merkusii) occur in India. They are distributed in Himalayas, north eastern India and some other parts of the country.
Some other species, found in India but not indigenously, are Pinus canariensis (Kashmir), P. caribaea (Assam), P. halepensis (Srinagar), P. massoniana, P. patula, P. pinaster and P. taeda (Kulu, Manali), P. radiata (Nilgiris) and P. thunbergii (West Bengal).
Plant body differentiated into roots, stem and leaves
Root of Pinus:
The type of root is tap root, tap root do not penetrate deep into the soil, root hairs poorly developed. Roots are covered with fungal hyphae, called mycorrhiza.
The fungi present in the ectotropic mycorhiza are the species of Amantia, Boletus,Clavaria and scleroderma of the class Basidiomycetes remains in close association with the roots.
It has been reported that in the absence of mycorrhizal association, the death rate of Pinus seedlings are considerably increased.
cylindrical, erect, woody and branched. The branches are monopodial and develop in the axil of scaly leaves.
In Pinus, the branches are dimorphic.
Long shoots or branches of unlimited growth.
Dwarf shoots or branches of limited growth.
Leaves are Dimorphic. Scaly leaves and foliage leaves.
Scaly leaves :
Dark brown, membranous, thin and small and present on both long and dwarf shoots. Do not help in photosynthesis. Helps to protect the young buds. They fall off as the branches mature. They have distinct midrib called as midrib.
Foliage leaves :
Green, acicular and needle like. Born only on dwarf shoot. A dwarf shoot with a group of needle like foliage leaves is known as foliar spur. The number of needles in a foliar spur varies in different sps. E.g P. monophylla is monofoliar with a single needle, P. sylvestris and P. merkusii are bifoliar with two needles, P. longifolia and P. gerardiana are trifoliar with three needles, P. quadrifolia is quadrifoliar.
The foliage leaves are photosynthetic and remains persistent for several years. The needle like nature of foliage leaves depict the xerophytic adaptation of these plants.
Internal structure of Pinus
T.S of root
The outermost, tightly appresed cells, gives out unicellular root hairs.
There 4-5 layers of parenchyma cells. Some cells filled with resinous substances.
The innermost layer having typical bands on their radial walls.
It is made up of 6-7 layer ofparenchymatous cells.
The roots are di-hexarch with radial and exarch vascular bundles. protoxylem bifurcates to form a Y shaped structure and a resin duct lies in between the two arms of protoxylem. The xylem consists of only tracheids; vessels however are absent. The phloem is composed of sieve tubes and phloem parenchyma, companian cells are absent.Pith poorly developed or absent.
Epidermis is made up of heavily cutinised cells.
outer sclerenchymatous hypodermis,inner parenchymatous cell with resin canals & filled with tannins.
Endodermis& Pericycle: indistinguishable.
Vascular bundles :
5-9 primary vascular bundles arranged in a ring. They are conjoint, collateral, open & endarch, tracheids are arranged in uniform radial rays.
Here cell parenchymatous filled with resin..
Old long shoot
The cambial ring continues to form increments of secondary xylem and secondary phloem every year. The xylem ring formed in a year is differentiated into Spring wood and autumn wood.
Spring wood – broad tracheids
Autumn wood- narrow tracheids.
Spring and autumn wood together consitutes the annual ring with the help of which the age of the plant can be determined.
Secondary xylem –
Secondary xylem is pycnoxylic. Tracheids in autumn wood is narrow and thick walled and those in spring wood is wider and thin walled.
Important characterstic of the wood of pinus is the presence of Bar of Sanio. They occur in the form of crecentric bars in between the pits. These bars are formed by the deposition of cellulose and pectin on tracheid wall. As the wood matures, the bars are separated from the pits, and the bars of adjacent pits are fused to form rims of Sanio.
Periderm or cork:
• Phellogen ( cork cambium) develops in the outer region of the cortex. The phellogen divides periclinally to cut cork cells ( phellem) towards the outer side and secondary cortex (phelloderm) towards the inner side.
As the stem increases in girth, the epidermis ruptures and the cork cells form a protective covering. The cork cells are impervious to water and they check transpiration from the stem surface
Structure of dwarf shoot:
It is similar to long shoot.
Epidermis: is single layered & cells are thick walled
Cortex : 4-6 layered parenchymatous cells filled with tannin cells & resin canals
Vascular cylinder : 6 conjoint, collateral, open & endarch vascular bundles. Medullary rays connect the pith to cortex
T.S of needle
Pith : parenchymatous. Show limited amount of secondary growth
• Epidermis- single layered, thick walled cells. Deeply sunken haplochelic stomata.
• Hypodermis- 2-3 layers of sclerenchymatous cells interupted by substomatal cavities.
• Mesophyll- 3-5 layered mesophyll tissue of polygonal parenchymatous cells with dense chloroplastrs and starch grains. Walls of the mesophyll cells give rise to many peg-like infoldings which increase the photosynthetic area of these cells. Only two resin canals in the mesophyll just below the hypodermis. Each resin canal has a layer of secretory epithelial cells surrounded on the outside by a sclerenchymatous sheath.
• Endodermis- with proper casparian thicknings. Pericycle- multilayered, made up of parenchymatous cells & some sclerenchymatous cells form T-shaped girdle.
Made of three types of cells;which form transfusion tissue.
Parenchymatous cells filled with starch
Albuminous cells :
occur in close contact with phloem cells. Parenchymatous cells with starch grains and proteins. Help in the translocation of nutrients from mesophyll to phloem cells.
Tracheidal cells: Tracheid –like occur close to xylem cells. Posses pits and helps in the conduction of water and minerals to mesophyll.
Vascular bundles –
No. varies in different species. Single in P. monophylla , two in P. roxburgii. Conjoint, collateral and open.
Narrow acicular form, thickly cuticularised epidermis, sunken-stomata, sclerenchymatous hypodermis. Peg-like infoldings of mesophyll cells and the presence of mesophyll tissue over the vascular bundles are some of the characterstic features of the leaves which indicate the xerophytic nature of Pinus.
They are Monoecious. The sporophytic plant body forms two types of spores – Microspores & megaspores which develop into male & female gametophytes respectivelly
Male cones :
• It develop on the axial of scaly leaf on long shoots. Male cones develop as groups just behind the apical bud. No. of cones in a group varies from 15(P. wallichiana) to 140(P. rouxburgii)
• It is a small oval structure, 2-4cm long & development of male cones start before the female cones.
• Single male cone consists of spirally arranged microsporophylls & no. varies from 60-150.
•Membranous, stalked & roughly triangular.
• The terminal sterile part is known as apophysis.
It bears 2 microsporangia at its abaxial side. Each microsporangium has numerous microspores. Except for few basal sporophylls, all are fertile.
• Uninucleate structure with outer exine and inner intine, exine expands on the lateral sides to form 2 balloon like structures known as wings.
Germination : greminate while still inside microsporangium.
• They are released at 4 celled stage i.e first & second prothelial cell, a generative cell & a tube cell. Disseminated by wind.
• Formed in groups of 1-4 in the axial of scaly leaves. Female cones takes 3 years to mature
• First year cone(1-2 cm in length, red green & closely arranged microsporophylls).
The second year cone(large, woody with sporophylls still compactly arranged).
The third year cone(become loose & sporophylls separate from one another due to elongation of cone axis.
• It consists of two types of scales: bract scales & ovuliferous scales
• Bract scale: small memberanous structure directly attached to the cone axis just below the ovuliferous scales. At maturity of the cone, the bract scales curve inwards so that seeds are dispersed easily.
• Ovuliferous scale: it is thick, large, woody & brownish structure, attached to the dorsal surface of the bract scale. It is roughly triangular in shape & its upper broad & thick part is known as apophysis. Two ovules are present at the base of ovuliferous scale on its dorsal surface & micropyle is directed towards cone axis
Life cycle of Pinus