Lesson 6 Pinus pinaceae - Habitat, Morphology and Reproductive Structure
Pinus pinaceae (Sallo):
Usually it is found in temperate dry habitat. I.e. inner range of Himalayas in Nepal .But grows through the Northern Hemisphere. It forms dense evergreen forest.
It is long, evergreen, perennial tree. Young tree of pinus gives a pyramidal appearance. The plant is saprophyte, it is differentiate in to root, stem and leaves. The stem is erect, aerial, cylindrical, woody, branched. Branches are dimorphic i.e. there are two types of branches in stem. Long branches and Short branches
The branches, which come from the main trunk, are called long branches. They show unlimited growth. Lower long branches are longer and upper long branches are shorter. Therefore, the plant appears as pyramid like.
The branches, which come from the main trunk and from long branches, are called short branches. They are very short they show limited growth. Short branches terminate into a cluster of green needles like leaves. Short branches are replaced in every 2 or 3 year.
The root is well-developed tap root system but does not grow very deep in soil. The taproot produces lateral root, which grows extensively. The root hairs are poorly developed root tap is covered by protective root cap. Root is covered by a kind of fungi called mycorrhiza. If it is not present, pinus dies. The relationship between the fungi and root is called symbiotic relation.
The leaves are also dimorphic. There are two types of leaves.
- Foliose leaf
- Scaly leaf
Foliose leaves: - they are green, long needle like they arise from short branches. They take part in photosynthesis. On the basis of number of leaves, the species are of following types.
- P. monophyla: Short branches having one foliose unifoliar spur.
- P. murkurii: Short branches having two foliose bifoliar spur.
- P. roburgtaii: Short branches having three foliose trifoliar spur.
- P. quadrifolia: Short branches having four foliose qoadrifoliar spur.
- P. wallichiana: Short branches having five foliose pentafoliar spur.
Scaly leaves: - they are scale like no green, dark brown in colour and are thin. They arise from short and long branches. They protect stem or branches by covering.
Pinus is monocious. The male and female reproductive structures are called strobili or cone. They are produced in different branches of same plant.
- They are formed in group behind apical bud of long shoot or Long Branch.
- They are small, 2-4 cm long and oval in shape.
- The central axis of male cone is surrounded by spirally arranged microsporophylls.
- There are about 60-150 microsporophylls in each male cone.
Microspores (Pollen grains):
- At the base of each microsporophylls there are two sac like structures called microsporangia.
- Within each microsporangium, numerous spores are formed called microspores.
- When microsporangial wall bursts, the spores or pollen grains come out and appears yellow mass called shower of sulpher.
- The male gametophyte is formed from microspores.
- The microspores are uninucleate, thick walled having two wings.
- The microspores are surrounded by three layered wall exine, exointine and intine. The exointine expands to form wings.
- Within microsporangium, microspore divides to form four celled stage microspore.
- After pollination, a tube called pollen tube is developed from four celled microspore. Within pollen tube two male gametes are produced form generative cell.
- The female cones are formed in-group of 1-4 and towards the tip of the long shoots.
- They are larger than male cones i.e. 15-20 cm.
- The central axis of female cone is also surrounded by spirally arranged megasporophylls.
- At the base of megasporophylls ovules are produced.
- Lower few megasporophylls are sterile having no ovules.
- Each megasporophyll consists of small membranous scales called bract scales and large, thick, hard, and triangular scales attached on the dorsal side of bract scale called ovuliferous scales.
- At the base of ovuliferous scale, two ovules are present on the dorsal side.
- Within the ovule the megaspore is developed.
- It is formed from megaspore within ovule.
- The megaspore divides multiple times to form female gametophyte.
- Archegonia are formed form peripheral cells towards micropylar region of female gametophyte.
- Each archegonium consists of 8 celled neck, one Venter canal cell and one larger egg.
After one year of pollination, fertilization takes place. Zygote is formed and then developed into embryo. The embryo consists of more than two cotyledons. New ovule is transformed into seed. Under favorable condition, seeds fall down and new plant germinates.
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