Gymnosperms-07-web - Gymnosperms 1 Gymnosperms Last lecture...

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Gymnosperms Gymnosperms Last lecture we discussed the importance of vascular tissue in nutrient transfer. Without vascular tissue, plants would have been small, like most bryophytes. Today we discussed the life-cycle of ferns, which was typified by the following features: Sporophyte dominant – Why has there been a shift toward sporophytes away from the dominant gametophyte found in mosses? Some speculate that evolution on land favors having two copies of genes. The reason for this is the harmful effects of UV radiation. If UV light causes a mutation in one gene copy in the sporophyte, there is still another gene copy. The damaging effects of UV radiation are probably not as great in aqueous organisms - i.e., algae, etc. separate gametophyte water necessary for fertilization, due to motile sperm, which is a limiting factor for ecotypic success Note that these are homosporous , unlike gymnosperms: Homosporous – refers to plants where a single type of spore develops into a bisexual gamete Heterosporous – plants in which the sporophyte produces two types of spores that differ in size and develop into male or female. As we move up the plant phylogeny (Figure 27.3) we find additional innovations that helped make land plants successful. Today we’ll discuss the gymnosperms, which arose roughly 250 mya. The gymnosperms have three major innovations relative to the seedless vascular plants: 1. Much reduced size and complexity of the gametophyte. The gametophyte is no longer free living. (A major trend in plant evolution.) 2. A shift from motile to non-motile male gametes. This removes the need for water during fertilization. (A major trend in plant evolution.) 3. The evolution of the seed, which is an embryo of the sporophyte in a protective packaged that includes a food supply. (Retained in angiosperms.) The seed, the last innovation, forms the basis for the name gymnosperm, which literally means ‘naked seed’. The gymnosperms are called ‘naked seeds’ because their seed is not enclosed in a fruit.
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