Primitive Plants at Skyrail

Skyrail Nature Diary: October 2013

This month we will be looking at primitive plants in Barron Gorge. For our purposes, this will refer to land plants which produce spores not seeds. So what is the difference between a seed and a spore? A spore is simply a single cell containing all the essentials for making a whole plant. Typically they have little or no drought resistance and require a certain amount of moisture to survive. A seed has a protective coat and contains stored nutrients together with the germ cell. Another important difference between the plants discussed here and seed producing plants is their life cycles. In primitive plants there are two life stages represented by two different individuals. One of the stages produces spores that grow into another plant without any sexual reproduction. This stage is known as the sporophyte. The plants that germinate from the spores the sporophyte produces reproduce and are known as gametophytes. Sometimes there are separate sexes, while other times both sexes are represented by one plant. In all land plants one of the life cycle stages is reduced. In mosses the sporophyte has no chlorophyll except in the Stoneworts and is practically parasitic on the gametophyte. In all other land plants it is the gametophyte that is reduced meaning all the trees in the rainforest are sporophytes. In primitive land plants with this strategy, the two life stages live separately. In the seed plants the gametophyte is so reduced that it is unable to live on its own. In the flowering plants this has been taken so far that the gametophyte is nothing more than a single cell with eight nuclei. Four of the nuclei disappear whilst three join together to provide nutrients to the fertilised nucleus, the future sporophyte.

Mosses are characterized by not having vascular tissues which means they are unable to distribute water and minerals to the entire plant body. They also lack a hard cuticle as well as wood and can therefore never grow very big. Since mosses lack adaptations to deal with lack of water, they are restricted to moist places. This, however, does not mean they are rare. In rainforests they are in fact extremely common and can be found growing on rocks, tree trunks and leaves. In the latter case, they usually occur together with fungi and are referred to as epiphyll. Sometimes epiphylls can take over a leaf completely. As a result, some plants harbor mites in special structures known as domatia in order to keep the growth of the communities at bay. There are three kinds of mosses - stoneworts, liverworts and true mosses. 

Tassel ferns, also known as club mosses, are the most primitive of the land plants that have vascular tissues. These interesting plants, together with the horsetails, dominated the first of the forests in the world. These forests were very wet and the oxygen content in the atmosphere was much higher than it is today. When the world cooled and dried out, these plants shrunk and retreated to moist areas where they are still found today in low numbers. In the rainforest they are usually found up in the trees as epiphytes. In the Northern Hemisphere they usually grow on the ground in swampy areas. The leaves of tassel ferns are quite unusual, completely lacking veins and midribs. Research has shown that the leaves of these ancient plants can best be viewed as leaf scars. 

Horsetails are contemporaries of the tassel ferns and were an important component in the world’s first forests. Horsetails are easy to identify. They have simple straight stems interspersed with whorls of simple leaves. The common name of these plants refers to the fact that they resemble the tail hairs of a horse. There is no difference between the sexes or their germ cells. The spores are produced in clusters of umbrella shaped structures known as sporangiophores. These structures are normally bunched together in large number on a cone shaped structure known as a strobilus. Horsetails normally grow in wet sandy soils. Some species of Horsetail have been declared as noxious weeds in Australia and the US state of Oregon. In New Zealand all horsetails are classified as “unwanted organisms”. 

Ferns are by far the most common of the primitive plants today and have more diversity than the other groups. Fossil evidence indicates that the earliest known seed plants were very similar in appearance to the ferns and are called seed ferns as a result. Some ferns grow into quasi trees, the tree ferns. The trunks of these ferns consist almost entirely of compressed leaf bases. Some grow over 10 metres in height. The spores are borne in clusters of sporangiophores on the underside of the impressive fronds. Each sporangiophore has a hinge mechanism that springs violently back when activated by the appropriate environmental cue, thus dispersing the spores into the surrounding area. The gametophyte is a tiny, short lived and heart shaped plant two cells thick that has both male and female parts on the surface. 

By Tore Lien Linde