The Science of Botanical Art

Ferns 

By Dick Rauh 

Originally appeared in The Botanical Artist, Number 37

 

The world of plants does not begin or end with flowering plants. Non-flowering plants provide their own perhaps more modest charms, but decided charms they are. Conifers belong to these groups as do mosses, club mosses and ferns, and although none approach the number of species of angiosperms, there is interest and diversity a-plenty to investigate and draw.

Ferns are my favorite among these ‘lower’ vascular plants. They are a delight to see, lining a woodland path in dappled sunlight, providing a green understory in the cool of a forest on a hot summer’s day. Without flowers or seeds, how do they grow and reproduce? How does their life cycle compare to the life cycle of flowering plants? Lacking visible stems or trunks what is it that we see when we look at a fern? 

Perhaps it would be best to look into some of the botany that governs the life of ferns, some processes that are present in all categories of green plants, others that are distinctive just to ferns. All plants have a life cycle that involves two stages or generations, one that bears the seeds or spores, the other that bears the gametes, the male and female reproductive units. The major difference between these generations is contained in the nuclei of each cell they contain. Each cell in the plant body that makes up the seed-bearing stage, called a sporophyte, contains all the genetic information for that plant. They are diploid, with matching double strands of DNA. The gamete-bearing stage, the one that has the egg cells and the sperm cells, have nuclei that contain only half the genetic information. These are single strands of DNA bearing groups of genes that carry characteristics from the parents. Because of the miracle of sexual reproduction, none of these single strands is exactly the same as another, although they carry codes for each specific character that determines a species. Fertilization is the joining of the two haploid halves of egg and sperm, to form a zygote, the first cell of a sporophyte, now with all of the genetic information.

Ferns produce no seeds, a much later manifestation on the evolutionary time line. The spores that we find on the back of certain fern leaves are contained in little sac-like structures called sporangia, which are often grouped in structures called sori and covered in tissue called indusia. The form of sori and their coverings vary with each genus, and provide a useful guide for identification. These spores have a hard covering and contain a single cell. A special reproductive cell division occurs within the spore, and it divides into four daughter spores, but now with only half the genetic information. These can be scattered by the wind, but generally fall at the base of the parent plant. Each daughter spore has the promise of germinating into a mature plant that bears the male and female organs to carry on the history of the species. Thus the life cycle of a fern consists of two distinctively different plant bodies. The one with which we are familiar; leaves with a wide variety of dissection, anywhere from a few centimeters to a few meters tall, called a fern sporophyte (phyte = plant, i.e. the plant that bears the spores or seeds). The fern gametophyte, on the other hand, is a much more modest fellow, a tiny, usually heart-shaped bit of green tissue barely the size of my thumbnail that sits in wet places below it’s parent plant, depending on moisture to allow the flagellate sperm to swim to the vaseshaped structure that contains the egg and thus fulfill the promise of fertilization. 

Flowering plants undergo a similar alternation of generations, except that the gametophyte, haploid stages are either hidden within the ovule and reduced to seven cells in case of the female, or in the pollen grain and reduced to three cells in the case of the male. What we see and love in flowering plants is the dominant, persistent sporophyte. 

Chances are we will never paint a fern gametophyte; so let’s look at the mature sporophyte. The most visible structure is the leaf, and with ferns we call that a frond. The petiole of a frond is called a stipe, and the midrib of the lamina (“leafy” part) is called the rachis. Often the stipe and rachis are covered with scales or hairs, an important characteristic for identification. 

This brings us to the language of dissection, terms that can be applied to any leaf, but because in ferns we are often dealing with species that have very subtle differences in leaf form we need to get down to the nitty-gritty. I will try to give an example of each type of dissection, and include a silhouette drawing. First there is the entire (without lobes or divisions) leaf, like the Hart’s Tongue (Phyllitis scolopendrium). Then comes the fern with many lobes, but the divisions between the lobes never reach the rachis. This is known as pinnatifid, and sensitive fern is typical. When the divisions do reach the rachis, each leaflet is separate, and called a pinna, and the fern is pinnate, as in the Christmas fern (Polystichum acrostichoides). The more complex dissection patterns are combinations of these two forms, and when the pinna is itself divided into separate segments these are called pinnules, and when pinnules are divided these are indeed called segments. 

Ferns do have stems and roots. Generally the stems are horizontal below ground structures called rhizomes, but sometimes these are vertical, usually producing plants with a crown of fronds. In some ferns the stems are horizontal and aboveground, usually with a furry covering and are known as stolons. The houseplant rabbit’s foot fern (Davallia sp) is an example. 

As we paint and draw ferns we discover a whole new range of greens, a new attention to the texture of the frond, the dissection of the leaf, the presence of scales, the form of the indusia, and the sori. These are details that make the modest non-flowering pteridophytes an engaging challenge.

  • Fern Lifecycle by Dick Rauh
  • Onoclea sensibilis by Dick Rauh