SelaginellaSpike Mosses Structure and Form
The sporophytes of Selaginella, the larger of the two major genera of living club mosses, are sometimes called spike mosses (Fig. 21.5). The approximately 700 species are widely scattered around the world in wetter areas, but they are especially abundant in the tropics. A few are common weeds in greenhouses. They tend to branch more freely than ground pines, from which they differ in several respects. The two most obvious differences are (1) their leaves each have a tiny extra appendage, or tongue, called a ligule, on the upper surface near the base and (2) they produce two different kinds of spores and gametophytes—an advanced feature referred to as heterospory. The seed-bearing coniferous and flowering plants discussed in chapters to follow are all heterosporous.
Reproduction
Sexual reproduction of spike mosses and ground pines both involve the production of sporangia, but spike moss sporangia develop on either microsporophylls or megasporophylls (Fig. 21.6). Microsporophylls bear microsporangia containing numerous microsporocytes that undergo meiosis, producing tiny microspores. The megasporangia of megasporophylls usually contain a megasporocyte that, after meiosis, becomes four comparatively large megaspores.
Each microspore may become a male gametophyte consisting simply of a somewhat spherical antheridium surrounded by a sterile jacket of cells within the microspore wall. Either 128 or 256 sperm cells with flagella are produced in each antheridium. A megaspore develops into a female gametophyte that is also relatively simple in structure. By the time this gametophyte is mature, however, it consists of many cells that have been produced inside the megaspore. As it increases in size, it eventually ruptures its thickened spore wall and produces
Chapter 21
microphyll sporophyte—
gametophyte sporophyll fertilization archegonium containing egg microphyll sporophyte—
gametophyte fertilization archegonium containing egg
sporophyll tetrad of spores
Figure 21.4 Life cycle of the ground pine Lycopodium, a homosporous lycopod.
tetrad of spores
Figure 21.4 Life cycle of the ground pine Lycopodium, a homosporous lycopod.
- Figure 21.5 Selaginella, a spike moss.
young sporophyte fertilization microsporocytes undergo meiosis, producing young sporophyte microsporocytes undergo meiosis, producing fertilization
microspore male gametophyte
Figure 21.6 Life cycle of the spike moss Selaginella, a heterosporous lycopod.
microspore male gametophyte
Figure 21.6 Life cycle of the spike moss Selaginella, a heterosporous lycopod.
several archegonia in the exposed seams. The development of both male and female gametophytes often begins before the spores are released from their sporangia. Fertilization and development of new sporophytes are similar to those of ground pines.
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