Answer to Question #18619 in Other Biology for namrathan
Reduction is necessary for sex reproduction because it provides maintenance of ploidy during generations. It prevents doubling of chromosome set number after each fertilization. Thanks to reduction gamete gets only one allele of a pair of alleles of the gene in parent organism.
There is no ploidy reduction during mitosis. Sister chromatides bear to the opposite sides of the dividing cell. Therefore genotype of the daughter cells is identical to that of the mother cells.
2) What is heterospory? Briefly comment on its significance. Give two examples.
Answer: Heterospory is the production of two kindes of spores: male (smaller ones, microspores) and female (larger ones, megaspores) by sporophyte of some higher plants. Microspores develop into male gametophyte with male sex organs and megaspores form female gametophyte with female sex organs. Corresponding gametophytes produce male and female gametes.
Heterosporous plants include several Lycopodiophyta such as Selaginallales and Isoetales, several Pteridophyta (Marsileales and Salviniales), all Gymnospermatophyta and Magnoliophyta.&
1. Heterospory makes possible division of labour between male and female gametophytes. Megaspores generally have supply of nutrients for gametophyte and then - embryo development. Microspores develops into small male gametophytes and have some devices for effective spread. Such division of labour facilitate and accelerate reproduction of heterosporous plants.
2. Gametophytes of heterosporous plants are frequently small and develop inside micro- and megaspores. It provides effective protection of gametophyte against adverse environmental conditions what also accelerate development.
3. Heterosporous plants are usually dioicous what promotes outcrossing.
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