The Life History of a Fern
The Life History of a Fern
Up to a hundred and eighty years ago or so, no one knew the details of a fern’s life history, and the secrets of the fern’s sexual life were not discovered until the invention of the microscope and improvements in laboratory technique enabled the botanists of the day to find out the mechanics of the fern’s sexual organs.
Because of this obscurity, the ferns and their allies were classed in a group of plants known as the Cryptogams, meaning ‘hidden marriage’: the ferns themselves were the ‘Vascular Cryptogams’, because their internal structure included a vascular system of vessels for conducting fluids.
Unlike the flowering plants, which elaborate their sexual processes within the flower, an extension of the vegetative part of the plant, the fern plant as we see it in the garden has no sexual mechanism at all. The fern plant is asexual, neuter, producingfrom its fructifications, which on germination do not produce another fern plant, but form an entirely different plant, a small green scale-like organism which is the bearer of sexual organs; and is an entirely separate organism from the fern plant as we see it. When fertilization has taken place a young fern plant is produced, asexual, neuter, a bearer of .
The botanist’s name for the fern plant is the sporophyte — the spore-bearing plant. The small green sexual plant, the prothallus as it is termed, is named by the botanist the — the plant bearing sexual organs. Thus we have two entirely different plants involved in We fern’s life history, the sporophyte or non-sexual generation and the or sexual generation.
This phenonemon is termed the ‘Alternation of Generations’ to embrace the complete life cycle of a fern.
If we break into the circle at the stage of the adult neuter fern plant, the plant we see in our gardens, say a Male Fern, as this probably is the ‘commonest’ species, and if we examine the fronds we will see on their backs rows of small brown scales. These scales are membranes, technically named indusia, which protect the spore-producing organs, tiny little stalked capsules, termed the sporangia.
Each sporangium is a watch-shaped capsule, on a slender stalk; there may be several dozen of these in each little heap in some species. The little heap is termed a sorus (plural sori).
The Life Cycle of Ferns
The Asexual or Sporophyte Generation
S. Young sporophyte emerging from embryo under Prothallus.
S1 Mature sporophyte or Fern Plant.
S2 Fertile pinnule with sori.
S3 Mature sporangium from sorus.
S4 Spores (x about 133).
Sexual or Gametophyte Generation
g Germinating spore, beginning of prothallus.
G1 Mature prothallus producing-
g2 Antherozoids or sperms and archegonium containing egg cell which when fertilized produces the new sporophytes.
Each capsule or sporangium produces an even number of spores, usually sixty-four. It is possible to see the sporangia with a fairly powerful hand lens, but the spores themselves are individually invisible except under a microscope, so minute are they.
In the Middle Ages fern spores were credited with magical properties enabling those with the requisite knowledge to become invisible. Such people were said to ‘have the gift of fern seed’ — a useful accomplishment which could be invaluable at times today.
A Male Fern
A. MALE FERN. Section through sorus. a1 Sporangium. a2 Ripe sporangium. a3 Sporangium dehiscing spores. v. vein. i. Indusium (x 166). s. Stomium.
B. Germinating spore. a. Apical cell (x 166).
C. Young prothallus from above. a. Apical cell (x 166).
D. Underside mature prothallus (x 6).
E. Section through prothallus. r. Rhizoids. a. Antheridia. b. Archegonia.
F. Ripe antheridium. s. antherozoid (x 200).
G. Nearly ripe archegonium. e. egg cell. c. neck canal, cells broken down (x 200).
When the spores are ripe the sporangium, by means of the contraction of a special chain of cells — the annulus — bursts and turns itself almost inside out, shooting the spores into the air to be wafted away by the wind until they fall to ground.
On germination, supposing the spores to have fallen on a congenial surface, at first a small thread of cells is formed; eventually cell divisions inclined to the axis of the thread cut off an apical wedge-shaped cell and further lateral divisions gradually build up the small green heart-shaped scale which is the prothallus, the independent plant bearing the sexual organs. An average size for a prothallus is about half an inch diameter. It is attached to theby fine root-hairs-the rhizoids-and it may grow or persist without appreciable increase in size for a year, in some species for two or three years, before anything else is seen to happen.
Eventually on the underside of the prothallus are developed the male and female organs, the motile male cells (the antherozoids) being formed in small groups of cells, the antheridia. The female egg cell is formed within a flask-shaped group of cells termed an archegonium — plural archegonia.
Fertilization of the female egg cell is effected by the tiny free-swimming male cells, tiny specks of life bearing within them all the necessary factors for reproducing faithfully the character of the neuter fern plant. They have to reach the egg cell by swimming through a film of water below the prothallus to the archegonia, down the necks of the archegonia until they reach the egg cells, of which there is but one to each archegonium. One antherozoid then penetrates to the centre of the egg cell and unites with it to form the ovum, as the fertilized egg cell is termed. From the ovum grows an embryo fern which eventually produces tiny fronds and roots, growing until a new fern plant is formed.
This alternation of sexual and asexual generations is characteristic of all fern plants, excepting a very few which have evolved a modified procedure mentioned later.
The sperms, or antherozoids, are attracted to the egg cells by a secretion of malic acid from the archegonia, so it will be apparent that the possibility of cross-fertilization orin ferns is a rare one because the path from the prothallus of one fern to that of another is very long indeed when compared with the distance between male and female organs on the one prothallus. Nevertheless, very occasionally hybridization does occur, and it can be effected by artificial means.
The fact that, usually, the male organs ripen before the female ones on the same prothallus may lead to wandering of the sperms to a more advanced prothallus near by in spite of the relatively great distance they have to travel.
If we now consider the further development of the fern plant, we see that from a single fertilized egg cell, by repeated cell division, is built up an ever-widening rootstock, bearing progressively larger fronds until the mature size is reached, the rootstock then being in the form of an inverted cone. Subsequently the rootstock grows more in the form of a cylinder, surrounded by the old frond bases, and terminating in a crown of fronds in the form of a shuttlecock, the youngest unfolded fronds protecting the growing point. The rootstock itself branches seldom, but often buds appear on the frond bases and these grow into mature crowns to form in due course a mass of crowns tightly packed together.
The frond of the Male Fern consists of a central axis which is considered in two sections. That section from the rootstock up to the blade is the stem, technically the stipes. The section supporting the blade or leafy portion is termed the rachis, from the lowest leaflets to the frond apex.
From the rootstock and from the frond bases roots emerge to anchor the plant and to obtain food in solution to be transmitted to the fronds for elaboration into tissue-building substances and also to maintain the frond in a firm plump condition.
The green parts of the frond absorb gases from the atmosphere and with the energy derived from light combine these with the materials in solution to make up complex organic compounds for use in further growth. This process is termed photosynthesis, which may be translated into everyday words: ‘making things with the assistance of light’.
The very existence of mankind depends on this property of plants of being able to produce complex body-building substances so inevitably and quietly. To effect the same processes by man-made apparatus is extremely difficult and costly in every way, involving great expenditure in fuel and human endeavour.
The stems are clothed with persistent brown scales; the rachis has fewer scales which are not persistent. The blade is compound, that is to say it is divided into segments like a feather, and therefore is termed pinnate: each leaflet is termed a pinna, plural pinnae. Each pinna is itself deeply cleft into many segments, but these clefts do not reach the pinna midrib and therefore the pinnae are not considered fully pinnate. This condition is termed pinnatifid, and the whole frond is described as pinnate, pinnatifid. Had the pinnae been completely pinnate the whole frond would be described as bipinnate.
These terms may seem to be leading us into deep waters, but after the first shock they will be seen to be very reasonable and concise.
The general outline of the frond is lance-shaped — lanceolate — fairly narrow, tapering towards the apex, tapering less sharply towards the base.
Towards midsummer on the backs of those pinnae which are fertile, will be seen two rows of small bodies on each segment. These are the sori, each covered with a membrane — the indusium. The shape of the indusium, plural indusia, is characteristic. In the case of the Male Fern it is kidney-shaped or reniform.
Under the indusia are little groups of sporangia, each containing forty-eight to sixty-four spores, and there are possibly a thousand of these groups on a fairly strong frond so that each frond can produce a few million spores. When one thinks of the numbers of Male Ferns in our countryside the total number of spores shed every season must be uncountable, and it is obvious that a very small percentage can survive. This is just as well, otherwise the whole surface of the globe would be a waving jungle of ferns in a very short time. Usually but one fern plant is formed from one prothallus, but cases are known where six or more embryos have developed and grown into mature plants from one prothallus.
It may be of interest to have some notes on the wonderful internal mechanism of the cell which lies behind the phenomenon of Alternation of Generations. All living things, animal or vegetable, start from one microscopic unit — the cell — within which is contained a highly complex body — the nucleus — roughly spherical in outline, suspended in the cell sap and protoplasm. Within the nucleus is the mechanism which carries all the characteristics of the particular species, and the instructions to the developing organism as to when and where to develop roots or shoots, leaves, conducting tissue, reproductive tissue and every aspect of the mature organism. Relatively simple in the case of plants, when one considers the higher animals and man himself, all his organs, every bone, muscle, nerve and brain, all differentiated and developed at just the right time from instructions contained within one microscopic speck of matter a thousandth of an inch in diameter, it makes the most complex electronic computer seem a primitive device.
Under the microscope the nucleus has a somewhat granular appearance which is shown up well by suitable stains. When the cell is about to divide the granular matter condenses into a long double thread coiled up within the nuclear membrane. This coalesces into, apparently, a single thread. This thread breaks up into a number of short rod-like bodies, the chromosomes, so called because they stain readily with suitable reagents making them easy to see under high magnification — the number of chromosomes being a fixed number for every organism: in the case of the Male Fern, one hundred and sixty-four, each of which carries its own particular group of hereditary factors. These chromosomes arrange themselves into a plane across the middle diameter of the cell, and each one splits lengthwise to form a pair.
An excessively fine spindle of threads forms in the cell with the chromosomes across the widest part of the spindle. Then the pairs of chromosomes separate, one set travelling to either end of the cell, where they become reunited into a thread and readopt the granular condition. In the meantime across the middle diameter of the old cell a new cell wall develops, so that there are now two cells both containing nuclei, each of which carries all the hereditary factors needed to form the mature plant.
When the spores are formed what is called the ‘reduction division’ takes place so that each spore contains half the number of chromosomes present in the cells of the fern plant. The prothallus resulting from the germination of a spore continues its growth with the half number of chromosomes, so that the male and female cells each have half the number of chromosomes found in the fern plant.
On fertilization the two half numbers unite to bring the number of chromosomes in the embryo to the full number again.
The prothallus orgeneration is said to be ‘haploid’, with a chromosome number of eighty-two; the sporophyte generation or mature fern plant is said to be ‘diploid’, with a chromosome number of one hundred and sixty-four — written 2n = 164.
It is possible by the use of various techniques, such as the use of radiation, or by the use of certain chemical compounds, such as colchicine, to induce spore formation without the reduction division; and when fertilization takes place in the diploid prothallus the embryo has double the normal number of chromosomes, written 4n = 328 (supposing the Male Fern was so treated). Such plants are termed ‘tetraploids’. The process can be carried further, turning tetraploids into ‘octaploids’. The most obvious effect of this chromosome doubling usually is the increased stature of the plants; in the case of flowering plants it sometimes causes doubling of the.
Research in the case of a flowering plant, the Giant Spearwort, has shown that this plant is the result of such doubling in nature, a very rare occurrence. It was shown by taking the small Ranunculus lingua, a plant some four or five inches high, and treating it with a similar technique to that outlined above, so as to double and redouble the chromosomes in succeeding generations, until the Giant Spearwort, Ranunculus lingua grandiflora, an octaploid, was actually produced artificially.
It is possible to cross a diploid with a tetraploid and thereby produce a plant with three times the normal number of chromosomes; such a plant is termed a triploid. These plants usually are sterile, as an even reduction division cannot take place, but two triploids can be crossed to form a ‘hexaploid’ which is fertile. Our nativeinterjectum, which is a hexaploid, is thought to have evolved in this way.
These techniques have been used to a great extent in the constant work carried out on hybridizing orchids.
It has already been mentioned that certain ferns have been able to make certain short cuts in their life cycle. One of these short cuts has been given the name ‘apogamy’, which means literally ‘without marriage’. This phenonemon is demonstrated in the Golden Scaled Male Fern, Dryopteris borreri. In such plants, when the spores are formed in some way the reduction division of the chromosomes fails to take place. When the spores germinate they produce a prothallus having the same chromosome number as the asexual fern plant. These prothalli form new fern plants by budding from the prothallus without the intervention of any obvious sexual process.
It seems possible that some form of reduction division may take place within the cells of the prothallus, but further research is needed to work out what actually happens.
The Ribbon Fern,cretica, is another example of a fern which demonstrates this abnormality, and its many varieties also carry the same character. Such varieties can be depended on to breed absolutely true to character.
The second short cut, or modification of the normal life cycle, is termed ‘apospory’, and in this case the sori, instead of containing normal sporangia, produce prothallial outgrowths in which some sexual process appears to take place. This phenomenon is shown by highly developed varieties such as Athyrium f. f. clarissima. In this case prothalli can be induced to grow from the abortive son bythe fronds in a suitable medium. Although the fronds are annual, deciduous, the prothallial growths will persist until the following year and produce young plants. Apospory has been demonstrated in the following Lady Ferns : Athyrium f. f. clarissima `Jones’, A.f.f. Uncoglomeratum `Stanfield’, the Male Ferns Dryopteris borreri cristata, D.b. Cristata apospora ‘Stanfield’, the Soft Shield Fern setiferum pulcherrimum — three separate forms — and in the Hartstongue, Phyllitis scolopendrium fimbriatum Drummondiae, and others. Unfortunately the plants obtained from aposporous prothalli are depauperate or deficient in some way.
Incidentally these abnormal variations in the fern life cycle, observed in varieties which were dismissed as monstrosities of no genetical importance by some botanists at one time, were brought to the notice of the scientific world by amateur fern-growers.