Fig. 284.
Antheridium of mnium with
jointed paraphysis at the
left; spermatozoids at
the right.
519. Structure of the moss capsule.—At the free end on the moss capsule as shown in the case of mnium in [fig. 280], after the remnant of the archegonium falls away, there is seen a conical lid which fits closely over the end. When the capsule is ripe this lid easily falls away, and can be brushed off so that it is necessary to handle the plants with care if it is desired to preserve this for study.
520. When the lid is brushed away as the capsule dries more we see that the end of the capsule covered by the lid appears “frazzled.” If we examine this end with the microscope we see that the tissue of the capsule here is torn with great regularity, so that there are two rows of narrow, sharp teeth which project outward in a ring around the opening. If we blow our “breath” upon these teeth they will be seen to move, and as the moisture disappears and reappears in the teeth, they close and open the mouth of the capsule, so sensitive are they to the changes in the humidity of the air. In this way all of the spores are prevented to some extent from escaping from the capsule at one time.
521. Note. If we make a section longitudinal of the capsule of mnium, or some other moss, we find that the tissue which develops the spores is much more restricted than in the capsule of the liverworts which we have studied. The spore-bearing tissue is confined to a single layer which extends around the capsule some distance from the outside of the wall, so that a central cylinder is left of sterile tissue. This is the columella, and is present in nearly all the mosses. Each of the cells of the fertile layer divides into four spores.
Fig. 285.
Two different stages of young sporogonium of a moss, still within the archegonium and wedging their way into the tissue of the end of the stem. h, neck of archegonium; f, young sporogonium. This shows well the connection of the sporophyte with the gametophyte.
522. Development of the sporogonium.—The egg-cell after fertilization divides by a wall crosswise to the axis of the archegonium. Each of these cells continues to divide for a time, so that a cylinder pointed at both ends is formed. The lower end of this cylinder of tissue wedges its way down through the base of the archegonium into the tissue of the end of the moss stem as shown in [fig. 285]. This forms the foot through which the nutrient materials are passed from the gametophyte to the sporogonium. The upper part continues to grow, and finally the upper end differentiates into the mature capsule.
523. Protonema of the moss.—When the spores of a moss germinate they form a thread-like body, with chlorophyll. This thread becomes branched, and sometimes quite extended tangles of these threads are formed. This is called the protonema, that is first thread. The older threads become finally brown, while the later ones are green. From this protonema at certain points buds appear which divide by close oblique walls. From these buds the leafy stem of the moss plant grows. Threads similar to these protonemal threads now grow out from the leafy stem, to form the rhizoids. These supply the moss plant with nutriment, and now the protonema usually dies, though in some few species it persists for long periods.