The Evolution Of Vascular Plants: The Ferns (1962)

when biologists study the anatomy of ancient plants they may use a technique called the fossil peel from a carefully prepared fossil organic materials can be transferred to a thin sheet of plastic for study under the microscope this peel shows the cellular structure of a plant stem in cross-section the plant lepidodendron is a distant relative of the modern Club mosses it grew more than 265 million years ago this fossil peel from the same period shows the cellular structure of ancient Fern roots in cross-section the study of plant fossils helps biologists understand how plants have evolved the evidence is incomplete but we can reconstruct the general outline of plant evolution by comparing fossils with living plants among the first plants were probably simple algae like chamid demonus these living fossils contain chlorophyll enabling them to convert the simple materials that bathe them into food and protoplasm the earliest land plants may have been primitive algae like fr from India to survive on land plants adapted in several ways to evaporation and other effects of air and sunlight most important in the development of all land plants Is the epidermis an outer layer of cells that retards evaporation here is the epidermis of a primitive land plant in cross-section showing one of the openings that permits gas exchange through the modification of tissues land plants have developed specialized organs leaves the more Leaf area exposed to sunlight the more food the plant can manufacture through photosynthesis the plant's main Axis or stem is another specialized organ finally there is the root system system providing Anchorage and absorbing water and minerals from the soil no one plant developed Roots stem and leaves all at once all of these specialized structures depend on the development of a conducting system a conducting system that permits fluids containing raw materials and manufactured food to flow throughout the plant these thick walled conducting cells form what is called the vascular system all plants with such a vascular system are called tracheophytes although a vascular system like this is found only in advanced plants such as the angiosperms the very ancient plants had less Advanced vascular systems the Moss pyrium has a primitive sort of conducting system although it is classified as nonvascular in pyrium this primitive conducting system is composed of elongated living cells the movement of fluid through such cells would be blocked by the end walls of each cell and it would be slowed by cytoplasm within the cells any flow of fluid would then have to take place by osmosis if we examine another primitive and ancient plant one of the tracheo phyes we can see some improvement the conducting cells of this plant have thin areas in the walls that permit some flow of fluid such a plant is cotum cotum does not have true leaves Roots part of the cotum stem grows underground and is called a ryome it can absorb water and dissolved minerals through microscopic hairlike processes called Riso from the Riso fluid passes through the outer stem tissues it is then transported to other parts of the plant through a primitive conducting system composed of dead cells called the xylm these xylem cells also give the plant rigidity other vascular cells carry the products of photosynthesis and are called flm These are living cells thus the vascular system conducts fluids from one remote part to another and it provides supporting tissues in the plant plants that rise above other species can receive more energy from the Sun the club mosses and the horse tails both have small leaves and Roots the evolution of roots and leaves represents an advance among primitive tracheo phyes the ferns are also tracho fights they show an even greater Advance with the evolution of large complex leaves the ferns in their many species occur in abundance some Gardens and conservatories have ferns from many parts of the world there are the giant tree ferns of the tropical forests this Fiddle Head is part of a tree fern from Hawaii the tiny free floating salvinia lives in water this delicate Maiden hair fern is native to Peru while the Exotic stagghorn Fern is from Australia the Holly Fern Grows in Japan somewhere on the margin or lower surface of a fern Leaf are rows of small dots these vary according to the species and are called sorry Sor are groups of tiny capsules which contain spores the spore producing plant is called a sporo the tiny capsules which make up each Sor are called sporangia sporangia disperse spores by ejecting them into the air the mechanism for dispersing spores can be seen in this microscopic view when the spores are mature the the sporangium bursts and stretches back like a catapult so that we may see it better the motion has been slowed down by placing the sporangium in glycerin spores are almost microscopic in size a tiny filament grows out from the Spore through a process of cell division this becomes a new plant not at all like the familiar Fern this plant is often called a prothallium on the underside of the prothallium are many Riso two kinds of reproductive organs are also found here male sex organs and female sex organs which produce sex cells or gametes it is called a gametoy the male sex organs are called antheridia mature sperms leave the antheridia only in the presence of water they swim by means of fella one of these swimming sperms May reach an egg in the female sex organs or aragonia when a sperm combines with an egg the fertilized egg or zygote is the first cell of the embryo this grows through a process of cell division the new plant which derives food from the parent gametoy is the new spor ofy generation in time it grows into the familiar fern plant with welldeveloped leaves called fronds a characteristic of fern plants is the way they spread by extending a system of underground stems or romes in many directions from various parts of the ryome grows a network of roots ferns like some of the more primitive plants include in their life cycle an alternation of generations a plant of the spori generation will have progeny often difficult to find in nature of the gametoy generation the gametoy generation reproduces sexually from the fertilized egg cell which it produces grows a different generation which reproduces asexually or by means of spores if we examine other plants we notice that among the more advanced the spori generation increases in size and importance it is in this spori generation that vascular systems have evolved the alternation of gener ations can be studied in the laboratory in a normal gy the male and female sex cells each contain a specific number of chromosomes such cells are said to be haid the zygote is formed through the union of a sperm and an egg cell from this develops a new plant each cell contains twice the number of chromosomes and is called diploid thus a normal spori is said to be diploid through experimentation we can study the effect of chromosome number on the alternation of generations a leaf is removed from a young sporium after a time most of the cells die all except for a few cells which begin to grow these cells develop not into a sporophyte but into a gametoy which is diploid instead of haid if self- fertilization occurs the resulting spori is tetrol instead of diploid that is it cells contain four times the number of chromosomes found in normal gametes if the procedure is repeated with a tetrol leaf a gametoy develops that is tetrol thus a simple difference of chromosome number is not what determines the alternation of generations other experimental work can be done with ferns here under sterile conditions a fern plant from the field has been stripped of its outer leaves what remains are rudimentary leaves which would normally take 2 or 3 years to mature when one of these is placed in the culture medium it shows signs of growth within a week chlorophyll develops and in time elongation takes place along with the formation of a Fiddle Head or Crow after about 6 weeks the croser uncurls revealing a mature leaf or frond the experimentor can therefore study the growth and development of fern leaves under controlled conditions a drop of this liquid contains Fern spores that have been exposed to intense radiation the spores are planted in a culture medium radiated spores are planted in another tube this culture medium contains an amino acid methionine in low concentration each tube is sealed after 8 weeks the two cultures are compared the tube without methionine shows gyes with a large number of tumors the culture containing methionin shows almost no tumors thus by inducing tumors in simple ferns biologists can study life processes which may be related to abnormal growths in animals in conclusion we may consider the Fern's highly specialized land plants they have Roots they have stems shown here in longitudinal section and they have leaves all of these specialized organs depend on the evolution of a vascular system while ferns are evolving so too are other vascular plants for a vascular system gives the land plant adaptive advantages it can spread and grow tall and thereby receive more energy from the Sun e