Significance of Homospory, Apospory and Apogamy

What is Apospory and Apogamy? Before we describe these terms, we must first understand what homospory and heterospory mean. Homospory describes the condition in which the asexual spores are all identical in structure and development. The opposite condition of heterospory, is that in which there are two spore forms giving rise to distinctly different gametophyte generations. This also occurs in the pteridophytes. The heterosporous condition must have been derived from the homosporous and also must have been one of the earliest steps in the evolution of the seed habit in plants.

Apospory and Apogamy

Apospory is the name used to describe the development of a gametophyte from a sporophyte without any spore production. Apogamy describes the phenomenon by which a gametophyte gives rise to a sporophyte without the union of gametes. Both conditions are very widespread among ferm and a single plant may show both conditions.

Gametophytes produced by apospory arise on the leaves in one of three positions. They may develop in place of spores in the sporangia, on the placenta in place of whole sporangia or along the margins or the tips of leaves. The aposporous gametophytes frequently resemble those normally produced by spores, both in form and function. They are thalloid, with rhizoids and develop antheridia and archegonia. However, they differ cytologically. The nuclei are diploid like those of the sporophyte. In most cases, such gametophytes give rise to a new sporophyte apogamously, but cases have been known in which union of diploid gametes from aposporous gametephytes may show sporophytic characteristics. They have been recorded as developing tracheids just behind the growing point and sometimes stomata. They have even been observed to develop sporangia.

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Apogamy can occur in aposporous sporophytes as mentioned abpve or in those normally produced by spore. In a number of cases, the gametophytes which give rise to apogamous sporophytes have been shown to be derived from spores which were diploid, i.e., were formed without a previous meiosis. The sporophytes. Sometimes, however, the gametophtes which give rise to apogamous sporophytes are normally haploid. In such cases, the resultant sporophyte is haploid, but can give rise to normal spores by duplicating the nuclear material in sporogenous tissues.

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An apogamously produced sporophyte may arise in one of several ways from the gamete. Any vegetative cell may produce it, or a component cell of the archegonium may be involved. This can be a cell of the venter or neck or the unfertilized oosphere itself. When the last is the case, the phenomenon is one of parthenogenesis.

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The conditions of apospory and apogamy tend to abbreviate the normal life cycle replacing spore production or gametic union by vegetative reproduction.