There are 92 known species of Genus Polytrichum.
This genus is abundant at damp places. Most of species exist in tropical areas of world and in cool temperate zone.
Polytrichum can be distinguish into false stem and leaves. True stem and leaves are not present in it like other bryophytes.
Stem of Polytrichum is of two type, an aerial stem and underground stem called rhizome. Rhizome is without any leaves and is underground. However it bears root like structures called rhizoids. Rhizoids are involve in absorption of water and nutrients for plant. Erect aerial portion of stem bears leaves.
Leaves of Polytrichum has no complicated structure. Upper surface of leaves contain lamellae which increase the surface area of leaf for light capturing. These are structures for photosynthesis. Structure of leaf is shown in figure.
Polytrichum Life Cycle:
Male and female plants of Polytrichum are separate in most of species.
Male gametophyte (n) contain antheridia. Antheridia are produced at the tip of leafy branch called antheridiophore. A cluster of Antheridia is produced at the tip of branch, this cluster looks like flower. Antheridia are intermingle with hair like structures called paraphyses. These paraphyses are somewhat involve in protection of Antheridia. By mitosis inside antheridia, antherozoids (sperms) are produced. Antherozoids contain flagella for movement. After release from antheridia, antherozoids move in water drop towards archegonia.
Female gametophyte (n) produce archegonia at the tip of branch called archegoniophore. Arhegonia has two portions i.e. neck and ventre. Neck contain neck canal cells and beneath these cells ventre canal cells are present. By the bursting of these cells, the passage to ventre become open, and material from these cells also attract antherozoids to archegonia. Egg is produced in ventre portion of archegonia by mitosis.
Antherozoids travels through water and enter into archegonia, where they fertilize the egg. Fertilized egg (Zygote) then develop into embryo. This is the beginning of diploid generation the sporophyte.
Sporophyte of Polytrichum:
Sporophyte is dependent on gametophyte. It develop from embryo within archegonia. It contains three portions foot, seta and capsule. Foot for attachment to gametophyte and absorption of food. Seta is there to support the capsule. Capsule is the structure where spores are produce. Capsule of Polytrichum has very complex structure.
In the center of capsule there is columella running through out the length. Outer to the columella inner air space is present, this space contain inner trebeculae cells. Outer to this is spore sac, bounded by inner and outer walls. Spore sac is surrounded by outer air space, which also contain trebeculae. This whole arrangement is covered with capsule wall which has two cells thickness. Capsule is covered by calyptra, which is a portion of gametophyte.
Sporophyte produce spores inside spore sac. Spores are produced by meiosis and are haploid. After release from capsule spores germinate at suitable place and develop protonema. A bud is developed on protonema, which give rise to gametophyte. Spores are of two types male and female. So these develop two types of gametophytes male and female accordingly.
There are two generations in life cycle of Polytrichum gametophyte (n) and Sporophyte (2n) which alternate with each other.