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The early land plants, like the early land animals, did not live far from an abundant source of water and developed survival strategies to combat dryness. One of these strategies is drought tolerance. Mosses, for example, can dry out to a brown and brittle mat, but as soon as rain makes water available, mosses will soak it up and regain their healthy, green appearance. Another strategy is to colonize environments with high humidity where droughts are uncommon. Ferns, an early lineage of plants, thrive in damp and cool places, such as the understory of temperate forests. Later, plants moved away from aquatic environments using resistance to desiccation, rather than tolerance. These plants, like the cactus, minimize water loss to such an extent they can survive in the driest environments on Earth.

In addition to adaptations specific to life on land, land plants exhibit adaptations that were responsible for their diversity and predominance in terrestrial ecosystems. Four major adaptations are found in many terrestrial plants: the alternation of generations, a sporangium in which spores are formed, a gametangium that produces haploid cells, and in vascular plants, apical meristem tissue in roots and shoots.

Alternation of generations

Alternation of generations describes a life cycle in which an organism has both haploid and diploid multicellular stages ( [link] ).

 The plant life cycle has haploid and diploid stages. The cycle begins when haploid (1n) spores undergo mitosis to form a multicellular gametophyte. The gametophyte produces gametes, two of which fuse to form a diploid zygote. The diploid (2n) zygote undergoes mitosis to form a multicellular sporophyte. Meiosis of cells in the sporophyte produces 1n spores, completing the cycle.
Alternation of generations between the haploid (1 n ) gametophyte and diploid (2 n ) sporophyte is shown. (credit: modification of work by Peter Coxhead)

Haplontic refers to a life cycle in which there is a dominant haploid stage. Diplontic refers to a life cycle in which the diploid stage is the dominant stage, and the haploid chromosome number is only seen for a brief time in the life cycle during sexual reproduction. Humans are diplontic, for example. Most plants exhibit alternation of generations, which is described as haplodiplontic    : the haploid multicellular form known as a gametophyte is followed in the development sequence by a multicellular diploid organism, the sporophyte    . The gametophyte    gives rise to the gametes, or reproductive cells, by mitosis. It can be the most obvious phase of the life cycle of the plant, as in the mosses, or it can occur in a microscopic structure, such as a pollen grain in the higher plants (the collective term for the vascular plants). The sporophyte stage is barely noticeable in lower plants (the collective term for the plant groups of mosses, liverworts, and hornworts). Towering trees are the diplontic phase in the lifecycles of plants such as sequoias and pines.

Sporangia in the seedless plants

The sporophyte of seedless plants is diploid and results from syngamy    or the fusion of two gametes ( [link] ). The sporophyte bears the sporangia (singular, sporangium), organs that first appeared in the land plants. The term “sporangia” literally means “spore in a vessel,” as it is a reproductive sac that contains spores. Inside the multicellular sporangia, the diploid sporocytes, or mother cells, produce haploid spores by meiosis, which reduces the 2 n chromosome number to 1 n . The spores are later released by the sporangia and disperse in the environment. Two different types of spores are produced in land plants, resulting in the separation of sexes at different points in the life cycle. Seedless nonvascular plants (more appropriately referred to as “seedless nonvascular plants with a dominant gametophyte phase”) produce only one kind of spore, and are called homosporous    . After germinating from a spore, the gametophyte produces both male and female gametangia , usually on the same individual. In contrast, heterosporous    plants produce two morphologically different types of spores. The male spores are called microspores because of their smaller size; the comparatively larger megaspores will develop into the female gametophyte. Heterospory is observed in a few seedless vascular plants and in all seed plants.

Questions & Answers

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Source:  OpenStax, Concepts of biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11487/1.9
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