Station 6: Clade Tracheophyta - Class Lycopodiopsida (Club Moss, Spike Moss and Quillworts)

The lycopodiopsids (also called the lycophytes or lycopods), like all plants, are members of the Phylum Tracheophyta but, like ferns, lack seeds. It is one of the oldest lineages of extant (living) vascular plants and is considered the basal group for all of the living trachaeophytes (modern plants including ferns, club mosses, gymnosperms and angiosperms). These species reproduce by shedding spores and have macroscopic alternation of generations. Most members of Lycopodiopsida have a sporophyte generation that is dominant. They differ from all other vascular plants in having microphylls, leaves that have only a single vascular trace (vein) rather than the much more complex megaphylls found in ferns and seed plants.

The genera Lycopodium and Selaginella are used in this lab as a representative of the class Lycopodiopsida.  Of the two genera examined, Lycopodium, is more primitive but they also represent a major change in spore organization through a structure called a strobilis.  In Selaginella not only will you examine this new structure, the strobilus, but the spores inside this strobilus will demonstrate change as well. 

Lycopodium sporophyte

The sporophyte of Lycopodium is the conspicuous part of the life cycle.  Once a zygote starts to grow it will form a Rhizome that gives rise to multiple shoots of Lycopodium.  These plants have true roots and leaves (i.e. they contain vascular tissue) although the leaves are very simple (they are microphylls - leaf evolution will be covered in a future lab).  A notable feature of Lycopodium is the strobilus.  The strobilus is a more compact reproductive feature that becomes more noticeable in Lycopoium as the spores develop.  The strobilus is examined in detail below.

Selaginella species are creeping or ascendant plants with simple, scale-like leaves (microphylls) on branching stems from which roots also arise. 

Unusually for the lycopods, which generally have microphylls with a single unbranched vein, the microphylls of Selaginella species contain a branched vascular trace. The plants are heterosporous with spores of two different size classes, known as megaspores and microspores.

Under dry conditions, some species of Selaginella can survive dehydration. In this state, they may roll up into brown balls and be uprooted, but can rehydrate under moist conditions, become green again and resume growth. This phenomenon is known as poikilohydry, and poikilohydric plants such as Selaginella lepidophylla are sometimes referred to as resurrection plants.  The resurrection plant is common in southwestern Texas and throughout Mexico.

Below is a video that shows a time lapse of the resurrection plant when put in water.

Selaginella Sporophyte

The sporophyte of Selaginella is very similar to that of Lycopodium, however a microscopic view will reveal some key differences, especially in the strobilus.

Selaginella Strobilus

The biggest differences between the genera Lycopodium and Selaginella are in the strobilus.  In Lycopodium the strobilus contained homosporous sporangia (all of the spores are the same size) while in Selaginella we see that something has happened to the spores in the sporangia and there are now 2 different sizes (Heterospory). 

The megaspores (the larger ones) are spores that are many times larger than the microspores.  The megaspore can be found in the megasporangium and are attached to megasporophylls.  These megaspores will give rise to the female gametophyte.

The microspores are housed in microsporangia and are attached to microsporophylls.  The microsporangia will give rise to the male gametophytes.

This difference in spores (heterospory) will play a key role in seed development in modern plants.  

Genus Isoetes, commonly known as the quillworts, is the only extant genus of plants in the family Isoetaceae, which is in the class of lycopods. There are currently 192 recognized species, with a cosmopolitan distribution but with the individual species often scarce to rare. 

Quillworts are mostly aquatic or semi-aquatic in clear ponds and slow-moving streams, though several grow on wet ground that dries out in the summer. The Quillworts are spore producing plants and highly rely on water dispersion. Quillworts have different ways to spread their spores based on the environment.

Quillwort leaves are hollow and quill-like. Each leaf is narrow, 2–20 centimeters (0.8–8 in) long. The roots broaden to a swollen base up to 5 mm (0.2 in) wide where they attach in clusters to a bulb-like, underground rhizome characteristic of most quillwort species. This swollen base also contains male and female sporangia, protected by a thin, transparent covering (velum), which is used diagnostically to help identify quillwort species. They are heterosporous.

Quillwort species are very difficult to distinguish by general appearance. The best way to identify them is by examining their megaspores under a microscope. Moreover, habitat, texture, spore size, and velum provide features that will distinguish them.

Below are some examples of quillworts.

This biggest story of the evolution of land plants is the development of vascular tissue but another big part of this evolution of plants is the evolution of the seed.  The story of of the seed will be told in a future lab covering the gymnosperms and angiosperms but part of that story needs to be told here because along with the evolution of the seed is the evolution of the structures that house those seeds.

In the bryophytes a simple capsule (sporangium) protects the spores and releases them when they mature but in the lycopods this structure is modified as a strobilus to contain multiple sporangia stacked on top of each other and arranged in a spiral fashion around the stem.  If we look ahead we will find that the female and male pine cones of gymnosperms are also arranged in a similar fashion.  This may not be coincidental and may represent an evolutionary trend that arose in the lycopods and continued on into modern plants meaning that strobilus of a lycopod could be a homologous structure to the pine cone of a gymnosperm. 

We are using terms like "may" and "could be" here because there is some debate about this.  Before we discuss the alternatives examine the sequence below to visualize the proposed evolution of moss sporangium to a gymnosperm cone.