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Anatomy of Plants


Bryophytes (Moss, Liverwort)

Bryophytes The bryophytes include liverworts and mosses. Most species of liverworts are "leafy" and look somewhat like mosses, but close examination shows that the body of a liverwort has distinct top and bottom surfaces, with numerous rhizoids (rootlike hairs) projecting into the soil. In contrast, a moss has a stemlike structure with radially arranged, leaflike structures. Rhizoids anchor the plant and absorb minerals and water from the soil. Because bryophytes do not have vascular tissue, they lack true roots, stems, and leaves. Instead, they have rhizoids, stemlike and leaflike structures (Figure 11).

Figure 11 Bryophytes
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In mosses, the gametophyte is dominant - it is longer lasting. In some mosses, there are separate male and female gametophytes (Figure 12). At the tip of a male gametophyte are antheridia, in which swimming sperms are produced. After rain or heavy dew, the sperm swim to the tip of a female gametophyte, where eggs have been produced within the archegonia.
Moss Life Cycle Antheridia and archegonia are both multicellular structures, and each has an outer layer of jacket cells that protects the enclosed gametes from drying out. After an egg is fertilized, the developing sporophyte is retained within the archegonium as an embryo. The sporophyte, which is dependent on the gametophyte, consists of a foot that grows down into the gametophyte tissue, a stalk (seta), and an upper capsule, or sporangium, where meiosis occurs and where haploid spores are produced. In some species of mosses, a hoodlike covering is carried upward by the growing sporophyte. When this covering and the capsule lid falloff, the spores are mature and ready to escape. The rings of "teeth" projected inward from the edge of the capsule allows spores to be released only at times when the weather is dry (when they are most likely to be dispersed by wind). When a spore lands on an appropriate site, it germinates. The single row of cells that first appears branches, giving an algalike sturcture called a protonoma. After about three days of favorable growing conditions, new moss plants appear at intervals along the protonema. Each of these consists of the rootlike rhizoids and the upright shoots of a moss gametophyte. The gametophytes produce gametes, and the moss life cycle begins again.

Figure 12 Moss Life Cycle
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Moss Embryo Sperms are released when the antheridium ruptures, thus allowing them to swim freely in a water film toward the archegonium. The zygote is the first cell of the new sporophyte just after fertilization. The zygote divides by mitosis into a multicellular embryo within the archegonium (see Figure 12 and 13). This is the crucial step that separates plants from algae. The embryo then inserts an absorbing organ called the foot into the female stem tip. The other end of the embryo grows up and above the female stem to form a stalk (seta) and sporangium (capsule) anchored in the old archegonium. Early in its development the sporophyte is typically green, but by the time it is mature it is usually non-photosynthetic and dependent on the gametophyte for water and nutrients. Within the sporangium special cells called sporocytes divide by meiosis to produce thick-walled haploid spores. In the more advanced plant species, the embryo is enclosed within the seed as shown in Figure 21.

Figure 13 Moss Embryo
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