Mosses

Introduction

The first group of plants that took advantage of the new territory, land, were the mosses and their allies, the liverworts and hornworts. Together, they are called the bryophytes. They are land plants, but do not have a vascular system (xylem and phloem), seeds, or flowers. The other land plants are the ferns and their allies (horsetails, whisk ferns), which have spores and a vascular system; conifers, which have a vascular system as well as seeds but no flowers; and flowering plants, which have a vascular system as well as seeds and flowers. Let's see how the mosses are different from the other land plants, as well as from lichens (a mutualistic association of a fungus and an algae and/or a bluegreen bacteria).

Moss growing along a small creek that flows
into Austin Creek, just above camp

Two Worlds

Land plants have a problem that aquatic photosynthetic organisms (such as algae) don't have: they live in two worlds. They live in the world of soil, where they get their water and nutrients. But they also live in the world of air, where they get their sunlight and carbon dioxide. These two worlds are very different, and in fact are almost opposites. The world of soil has no sunlight, but lots of water. The world of air has lots of sunlight, which tends to dry up all the water as well as dry up the plant. Land plants had to develop means of coping with these two opposites, as well as connecting them. If you are the plant, having water in your roots does not help you much if you cannot transport it to the leaves which have the sunlight for photosynthesis; making sugars in the leaves do not do you much good if you cannot transport them to the root for storage. Land plants eventually solved this problem by developing structures such as roots, leaves, a stem to connect them, and a vascular system to transport nutrients, water, and metabolic products such as sugars and hormones.


"leaves" of moss	cells in the "leaf"	another view of "leaf" cells

The first land plants, however, did not have these systems. Mosses started the march onto the land by developing primitive systems to deal with the problems of living on land, but do not have fully developed roots, leaves, stems, and a vascular system. Mosses absorb their water and nutrients directly into their bodies, not through their "roots". Instead of roots, they have rhizoids, which serve to stabilize the moss but do not have a primary function in water and nutrient absorption. They lack a vascular system both in their rhizoids and in their above-ground parts. They also lack lignin, which is a complex compound that stiffens the stems of higher land plants. This is why mosses are so soft. Their "leaves" are actually single-cell thick leaf-like appendages. True leaves have several layers of specialized tissues. The lack of vascular tissue forces mosses to be rather short, because they cannot transport water, nutrients, and metabolic products.

Mosses do have a property that vascular plants do not have: they can dry out and still come back to life. Lacking roots, and therefore a ready way to obtain water all the time, mosses had to adapt to a feast or famine approach to water. Mosses can dry out to a remarkable degree, yet spring back to life with just a single sprinkling of water (this is rather fun to do when you see dryed out moss on a hike). Higher plants would die with a similar degree of desiccation.

Alternation of Generations

All plants have an alteration of generations. The two generations are called the sporophyte generation (generates the spores) and the gametophyte generation (generates the gametes, the egg and sperm). In mosses, the gametophyte generation is the larger, more easily seen generation. It is the leafy, green part that we think of as a moss. It produces the gametes, the egg and the sperm, which create the sporophyte generation.

The sporophyte generation has three parts, the foot, the seta, which is the small, usually brown slender stalk (see illustration at right) that sticks up beyond the green, leafy moss, and the capsule, which is a bulb at the tip of the seta. The capsule contains the sporangium, which contains the spores.

For a great illustrated explanation of the life cycle of a moss, see the University of Auckland, New Zealand, website.

Mosses, then, are land plants without a vascular system, seeds, or flowers, with a dominant gametophyte generation.

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