Plant Kingdom of Class 11
Occurrence : Grows in cold, shady and moist places.
External morphology : The main plant body is sporophyte which is differentiated into root, stem and leaves.
Root : Small, wiry adventitious roots, arising from the stem (rhizome)
Stem : Modified into rhizome, subterranean, creeping, oblique, dichotomously branched and covered with thin brown hair called ramenta.
Leaves : Large, bipinnately compound, differentiating into frond and stipe and showing circinate vernation.
Dryopteris : Habit sketch
♦ Root is diarch and exarch.
♦ Stem has dictyostele consisting of a number of meristeles which are amphicribal. Xylem mesarch.
♦ Vegetative : Takes place by fragmentation and adventitious buds.
The main plant body is homosporous sporophyte producing spores in sporangia present on the sporophylls.
The sporangia are produced in groups called sori (1-6 sporangia/sorus) which are submarginal, linear and discontinuous.
The sporangia are leptosporangiate.
The sori are surrounded by kidney shaped true indusium.
The sporangia arise from placenta of leaf.
The mature sporangium has a stalk and a biconvex capsule.
The capsule has a single layered thick covering called annulus which is incomplete at the base on one side, the portion being called stomium.
Each sporangium encloses 8 or 16 spore mother cells which after meiosis term 32 or 64 haploid spores.
Spore dispersal is due to xerochasy of stomium.
Dryopteris : A. Part of a sporophyll with sori, B. T. S. of sorus
Dryopteris : One sporangium
Spore in the first cell of gametophytic generation, which on germination forms a multicellular, green coloured heart shaped dersoventrally flattened structure called as prothallus.
Prothallus is monoecious and protandrous with an apical notch and unicellular rhizoids at the base.
Archegonia are found around the apical notch and antheridia among the rhizoids.
A mature antheridium has 3 celled sterile jacket (2 ring cells and 1 cap cell) and produces 32 spirally coiled multiflagellated antherozoids.
A mature archegonium is flask shaped structure with a neck and a venter. Neck has single binucleate neck canal cell and venter has a large egg and a small venter canal cell.
Detailed structure of a prothallus showing position of archegonia, antheridia and rhizoids
Antheridia A. At dehiscence B. Young antheridium C. One antherozoid
Water is essential for fertilization. Neck canal cell and venter canal cell disintegrate and secrete mucilaginous substances which is rich in malic acid, which attract antherozoids. On fusion diploid zygote (2n) is formed.
A. One Archegonium. B. Archegonium with male gamete in its neck
Young Sporophyte : The zygote divides mitotically to form an octant. Upper epibasal tier forms stem and leaf and lower hypobasal tier forms root and foot. Young sporophyte derives its nutrition from prothallus. Hence we can say that young sporophyte is dependent on gametophyte. Although mature sporophyte is independent.
Topographical representation of life cycle of fern