Bryophytes

The Flora of New Zealand follows a general thread of evolutionary trends in plants. The next section of the course covers what are commonly called ‘lower’ plants, Bryophytes. There are three main classes within the Bryophytes, Bryopsida (or Musci or Mosses), Hepaticopsida (Hepaticae or Liverworts) and finally Antheropsida (Anthocerotae or Hornworts). We will not be covering Antheropsida (hornworts) in this text.

Bryophytes are all deemed to be less morphologically and physiologically advanced or complex. We are going to find out why!

There are several key features that define plants; the presence of chloroplasts, vacuoles in the cells, cellulose in the cell walls, the ability to photosynthesise etc. When we examine both liverworts and mosses closely we find that while they have chloroplasts and many other higher plant features, they differ from large plants in several key characteristics. They have no ‘veins’ in their leaves or stems, they don’t have a waxy cuticle over their leaves, they don’t have any flowers or seeds and often very little in the way of roots, and they are generally quite small.

Have you ever wondered why?

The explanation involves a little bit of plant physiology. Now don’t panic…keep calm and read on…

The easiest and most energy efficient way to move molecules in living systems is by diffusion. This simply means that substances move from an area of high concentration to low. So, for example, water molecules move along a gradient from high water content to an area of low water content. Similarly sugars produced from photosynthesis move from areas of high concentration to areas of low concentration. But, this is only fast and efficient over very small distances (about 6 micrometers) – so the organisms that rely wholly on diffusion (i.e. they lack a transport/vascular system) are size limited, like liverworts and mosses.

Size limitations don’t just apply to the height of a plant, they also apply to the thickness of the ‘leaves’, or to put it a little more scientifically, the depth and complexity of tissue. When you look at mosses and liverworts you will see that they are mostly only one cell thick. There are areas that are slightly thicker but we’ll cover those when we look more closely at the morphology.

As mentioned earlier, both liverworts and mosses lack a waxy cuticle. The cuticle is the major means for minimising water loss from leaves in higher plants so bryophytes have no means of preventing the evaporation of water from their surfaces. In dry atmospheres there is rapid and uncontrolled water loss (think about the concentration gradient of water molecules) from the plant surfaces. The result is that bryophytes are also habitat limited; in other words, they are confined to damp and moist areas where there is a high humidity.

Another highly significant factor is the differences in the life cycles of bryophytes and higher plants. Each cycle is divided into two sections, one where the gametes (sex cells) are formed and the other where the spores are formed. The ‘plant bit’ that produces gametes is called the gametophyte (gameto = gamete and phyte = leaf, so it is the ‘leaf’ bearing gametes). The ‘plant bit’ that produces spores is called the sporophyte (sporo = spore, phyte = leaf, so it is the spore bearing leaf).

The diagram below is a generalised outline of the life cycle of liverworts, but is basically the same for mosses. The dominant generation is considered to be the main body of the plant, usually the one that is most visible, and provides the nutrition for the plant. Notice that it is the gametophyte that is the most visible and in the case of liverworts and mosses, is usually a permanent plant and it provides the nutrition for the dependent sporophyte. In mosses, liverworts and hornworts the dominant generation is the gametophyte. As we follow the evolutionary trends this changes and the dominant generation becomes the sporophyte (ferns) until in angiosperms the gametophyte generation is reduced to merely a small group of cells.

Figure 1. Typical thallose liverwort life cycle showing haploid and diploid stages. The same generations are followed in mosses. Adapted from Raven et al, 1999.

Fertilisation is carried out when the sperm and egg cells unite. In the case of both mosses and liverworts the sperm are flagellated and must ‘swim’ through water (usually rain drops) to the egg cell. This is another factor that limits the available habitats.

Now let’s move on to the Hepaticae (liverworts).