Wednesday, 19 May 2021

The heterosporous species are always dioecious

The heterosporous species are always dioecious



 In the latter, development of gametophyte is independent. external environment, whereas in the former, it homosporous pteridophytes the prothallus is branches. In species of Lycopodium, the prothallus simple, green and heart-shaped or has aerial gametophyte in different genera. In most of the the microspore gives rise to the male gametophyte (e.g.. Lycopodium, Selaginella. Equisetum) and the megaspore to the female gametophyte.

 There is much variation in the shape. The heterosporous species are always dioecious. The sex organs are embedded in the prothallus Pteridop is vulnerable to environment. is of the following three types. (i) In L. cernum it is very small and inconspicuous but autotrophic. In species like L. clavatum, L.

 Obscurum and (ii) L. selago, the prothallus is tuberous, subterranean and sporophytic. (iii) In some epiphytic species (e.g., L. phlegmaria) the whole prothallus is heavily associated with endophytic fungal hyphae which show symbiotic association.

 Most of the homosporous species are monoecious, i.e., antheridia and archegonia are borne on the same prothallus. They are protandrous of protogynous. Some homosporous forms (e.g.. Pterb Pteridium) are also dioecious. or are projected (e.g., Psilotum, Pteridium).

(ii) Gradate sorus : in this type the central or distal part of the sorus has mature/older sporangia and the peripheral or proximal part has immature younger sporangia (e.g., Marsilea, Cyathea; Fig. 4B). (iii) Mixed sorus : Here mature and immature sporangia are arranged irregularly (e.g., Pteridium, Pteris, Adiantum, Polypodium).

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