Cycas as the relic of past Cycas is an evergreen plant which looks like a palm. It has unbranched stem and large compound leaf. It exhibit phylogenetic relationship with pteridophyte. Its evolutionary characters are
(i) Slow growth.
(ii) Shedding of seed when the embryo is still immature.
(iii) Little secondary growth and manoxylic wood.
(iv) Leaf like megasporophyllus.
(v) Flagellate sperms even when pollen tube is present.
(vi) Persistent leaf bases.
(vii) Circinate ptysix.
(viii) Arrangement of microsporangia is well defined archegonia.
Heterospory, i.e., production of two types of spores smaller microspores and larger megaspore was first reported in Selaginella a pteridophyte. In Selaginella, the smaller microspores are destined to produce male gametophytes and the larger megaspores to female gametophyte.
The male gametophyte produces male gametes, whereas the female gametophyte produces archegonia and also provides nourishment to the developing embryo.
The phenomenon of heterospory, thus lead to the reduction of gametophyte, in situ germination of spores, retention of megagametophyte in the megasporangia and finally to the seed development. Thus, the heterospory in Selaginella forms the base for seed habit development in gymnosperms.
The life cycle of fern (Dryopteris) clearly shows the alternation of generation. The gametophytic stage $(n)$ alternates with the sporophytic stage $(2 n)$ figure given shows its complete life cycle.
Prothallus The prothallus of the fern is multicellular, free living, thalloid, haploid and autotrophic structure. It develops from the spores produced by sporophyte after reduction division. These spore germinate with a germtube with an apical cell and forms a filament of 3-6 cells and one or two rhizoids at the base which later develops into gametophytic plant.
The male and female gametophytes of pteridophytes and gymnosperms different from each other are
| Male Gametophyte of Pteridophyte | Male Gametophyte of Gymnosperm |
|---|---|
| A distinct male gametophyte may not be present. | A male gametophyte is always present. |
| It contains an antheridium. | An antheridium is not found. |
| Male gametes are flagellated. | Male gametes may or may not be flagellated. |
| Male gametes reach the female gamete by swimming in a film of water. | Male gametes reach the female gamete through a pollen tube. Water is not required. |
| Female Gametophyte of Pteridophyte | Female Gametophyte of Gymnosperm |
|---|---|
| A distinct female gametophyte may or may not be present. | A distinct gametophyte is always present. |
| It is largely independent.. | Female gametophyte does not leave the parent plant. |
| It is not enclosed is an ovule. | It is enclosed inside an ovule. |
Mycorrhiza (Myco's $=$ fungus, rize $=$ roots) is a symbiotic association between fungus and the roots of vascular plants. The fungus colonizes the roots of the host either intra or inter cellularly. It helps in the nutrient absorption from soil for the plant. Mycorrhizal associations are present in conifers, i.e., Pinus, Cedrus, Abies and Picea.
Coralloid roots is develop in Cycas. It is produced in clusters at the base of the stem and protrudes out over the ground. It is dichotomously branched and greenish in colour. It contains algal zone in cortex. This algal zone contains blue green algae like Anabaena and Nostoc which grow in symbiotic association with coralloid roots.