Lecture 13: The Sex Life of Plants PDF

Lecture 13: The Sex life of plants ● What happen after the publication of Origins? ● Darwin suffered from mysterious illnesses in his later life. ● Usually, the theories of his illness are the area of expertise of the person who put forward the theory. ● He goes about his scientific works. His next...

Lecture 13: The Sex life of plants ● What happen after the publication of Origins? ● Darwin suffered from mysterious illnesses in his later life. ● Usually, the theories of his illness are the area of expertise of the person who put forward the theory. ● He goes about his scientific works. His next subject of research ● He has not finished his BIG book. Origins is just a summary. ● Darwin’s home and his garden were his laboratory to research ● Emma and Charles Darwin, 1863, An appeal against steel vermin traps 🡪 deemed inhumane to animals ● ● ● ● ● o Only publication Darwin did with his wife A side project, a hobby – Orchid. Origins are being debated now. Darwin’s critics claimed that natural selection could not explain the details of living things. The delicate curves and structures of flowers was one example. Beauty of nature is like the divine of design (extremely detailed) Orchid collecting was becoming a major Victorian pastime Large network of steamboats bring about all kinds of things from other places, one of them is orchids. ● Linnaeus (1758) knew of a hundred species of orchids, but by 1860 there were 433 known genera with 6,000 species. ● By Darwin’s time, expand from 100 to about 6000. Huge explosion in numbers Why Darwin chose to examine orchids ● Darwin’s critics claimed that natural selection could not explain the details of living things. The delicate curves and structures of flowers was one example ● Orchid collecting was becoming a major Victorian pastime ● Linnaeus (1758) knew of 100 species of orchids, but by 1860, 6000 species were known Orchids (Flowers) ● Some believed they were created for their beauty to human eyes. Darwin knew this could not be true. o Some flowers appeared only at night and others in parts of the world uninhabitable by human beings. ● Other people thought some biological structures are just useless. ● Objection that it was created for the beauty. ● It cannot be the function for flowers. Flowers are there without people. Some only exist at night. Cannot be their reason for existence. ● Darwin believed their beauty had a purpose ● He believed that there must be a reason why they are there. Darwin had greenhouses or hothouses ● Darwin was able to do some kind of science at home such as botanical experiments. ● Grow orchids in his hothouse. Heated by steams. Next book Darwin published: Fertilisation of Orchids (1862) is the first of Darwin’s books of supporting evidence for the theory of evolution by natural selection. (orchid on cover, purple) Book’s name: On the various contrivances by which British and foreign orchids are fertilised by insects and on the good effects of intercrossing ● First book published after origin of SPECIES (1862) ● Why publish this book? Why not on human evolution? After his most famous book. ● Why bother publish something that is only his hobby. ● How do plants fertilise? Earlier theory: plant self-fertilisation (however this means that there will be clones, no change so no evolution, don’t change with environment will be doomed) ● Darwin’s theory suggest sex. Plants originally came from water and later colonise lands. Have problem on how to fertilise plants that are not on water. Sex cells can literally swim across to another land but cannot do that when it is dry. So, plants must develop seeds etc. to propagate in dry land. ● Why does Darwin’s theory need sex? Darwin is convinced that life cannot just go on with things budding of from themselves (self-fertilisation) or mating with themselves. Hermaphrodites. He is convinced that without any variation a lineage of such family will be identical and unchanging. There will be no variation for natural selection to filter out changes to help them adapt to a changing world. Therefore, he is convinced that is why sex is universal as it allows mixing. ● The stress tends to be on genetics in todays’ biology. Most of the variation in this world is caused by sex and not mutations. o Almost all organisms must reproduce sexually. Every time they do so they create enormous variation. ● Darwin is theoretically bias that flowers must have sex too. Bias against the view that flowers only mate with themselves. ● “On the various contrivances by which British and foreign orchids are fertilised by insects, and on the good effects of inter-crossing.” (1862) o Contrivances = adaptions – they are structures that look like they are designed to do something. ● Through this book, he can show that his theory works in its smallest detail level. ● Darwin to John Murray: “I think the little volume will do good to the ‘Origin’ as it will show that I have worked hard at details.” Parts of flower ● The fact that plants have different sexes had only been established around 60 years before. The male organ, the anther, produces pollen which must reach the pistil or female organ for the plant to produce fertile seeds. ● But they thought plants did this to themselves. o Pollinate themselves ● Darwin convinced that if plant self-fertilised, they will be doomed. Because they would not vary. He looked for ways to disprove this. ● How the pollen is transferred varies. Sometimes it is simply blown by the wind from one plant to another. But in most orchids the pollen is so firmly embedded that it cannot fall out because it was enclosed in a bundle called Pollinia. This was a mystery. How could orchids be pollinated? ● Orchid is a particular mystery. No one has seen an insect pollinate a flower. That is why people believe it did not happen. Darwin convinced that this cannot work. Early Purple Orchid (Orchis mascula) ● He starts to analyse and even dissect flowers to figure out their structure. ● Called Pollinia ● Collated in bundles. Pollen packets. ● He experimented by molesting the flowers. ● He inserted pencils into the flowers, to cause them to sexually react. o Tried to pollinate them artificially with their own pollen and other experiments ● He was imitating what happen if an insect gets into a flower to get into the nectar at the back. o The pollen has sticky part that stuck to the pencil. o After it dries a little, it bent forward, ‘act of depression’. If its bent forward, when you bring the pencil to another orchid, it will be in a right position to pollinate the other orchid. Works for pencil so presumably works for insect. (also used Catasetum from South America) ● He also experiments by covering orchids, the one uncovered were losing pollen packets at night. But the one covered, no pollinia missing. The one uncovered were fertilised. He was convinced that it must be insect. The cause is invisible, a very Darwinian problem. o Why would all these complex structures be there if they can self-fertilise? o What are these specific structures for? Moths ● Twenty-three species of moths had been found with orchid pollinia attached to their proboscis ● He found out that many moths were found with pollinia stuck all over them. ● This is called coevolution or coadaptation or friends with benefits (nowadays) o Need help from others ● Darwin showed that pollinia are adapted for removal by insects as they probed the flower’s depths for its nectar. As insects did so the pollinia became glued to the insect and detached as it withdrew and flew away. The position of the firmly attached pollinia was also just right to fertilise the next orchid visited by the moth when it again probed for a drink of nectar. ● He showed that pollinia are adapted to be moved by insects ● Darwin argued that structures of flowers have previous features. Flowers are modified leaves. What difference does it make? ● Darwin calculated that some orchids bear between 6000 and 186000 seeds per plant. If one’s seeds successfully grew, the great-grandchildren of the plant will cover the whole area of the Earth. ● Orchids make thousands of seeds. If each of the seeds live, the rate at which they reproduce can cover the whole earth but they obviously do not. o That means they face catastrophic destruction. That means they in effect face incredible competition. Only a few made it to survive. o Selection pressure is high, any little difference that helps it to survive will most likely be passed on ● Therefore, severe checks prevent orchids from reproducing to this extent. Darwin was not sure what these checks might be, but he could conclude that any naturally occurring differences that allowed a plant to reproduce successfully were likely to proliferate. ● Variations are important because they can improve an offspring's chance of survival. o Different flowers have sex with different flowers (only then then can produce variation) ● In Darwin’s days, it was believed that flowers reproduce by themselves 🡪 no variation o Male and female reproductive organs of flowers are near to each another, so the flowers can self-fertilise. ● Darwin found out that on the insect’s head, legs, etc., contain pollen which then carries it to another flower. o Darwin then realised that insects are full time insect disseminator for flowers. ● Flowers evolved to ensure that insects can transfer their pollen to another flower. ● Before Darwin days, there is no need to explain why it is so complicated because they are designed by God to be that way. ● Evolved structures to ensure that plants do not mate with themselves. ● There is only one way to ensure that the insect can get to the nectar and this way ensures that the pollinia lands and sticks onto the insects. ● Flowers exists is to tempt animals (bat, bird, insects) to pollinate the flower The long nectary of the Angraecum sesquipedale (an orchid from Madagascar) ● Darwin predicted there must be a moth where the tongue is long enough (10 inches long) though no one has ever seen it. He was ridiculed by his critics. o “in Madagascar there must be moths with proboscis capable of extension to a length of between ten and eleven inches!” ● A R Wallace 1867 convinced that Darwin must be right. He predicted this moth. o “That a moth exists in Madagascar may be safely predicted; and naturalists who visit that island should search for it with as much confidence as astronomers searched for the planet Neptune, -- and they will be equally successful” ● This flower has a reduced species that can drink from it ● Insects that drink the nectar from this species can only go to the same species 🡪 dedicated pollinator. ● Specialised but may not be a good thing, because for example if the flower disappears due to cats, the moth cannot survive. Discovered in 1903 Xanthopan morgani ● The moth discovered after Darwin’s death in 1903 ● Chicken and egg problem. The long tongue first or the long nectarine? Incremental change. 2 species coadapted with each other that they have a special relationship o This kind of relationship is good in a sense, these moths will definitely look for these orchid, but if these moths are extinct then it is disastrous ● This orchid must have ancestors with normal length nectary. ● Evolutionary arms race. ● Gradual evolution of structure. When nectarines grow slightly longer, it should be more beneficial for evolution, means that fewer insects can reach it. ● Reduced number of species that can pollinate this. ● A case where only one moth can possibly pollinate this kind of flower. ● If the moths disappear, the plant is doomed. Will go extinct. Like Pandas, they only eat bamboos. ● Conclude that flowers have just the right structure to guide insects into the right position to cause the pollinia to stick in this way and to receive pollen from visiting insects. o Plants had as many intricate adaptations as animals and their adaptations were cobbled together from pre-existing elements by natural selection. (endless series of variations) o The book pioneered the science of pollination ecology. o Orchids all share the same blueprint. ▪ Orchids come from same ancestors ▪ “Orchids exhibit an almost endless diversity of beautiful adaptations, a part which originally served for one purpose, by slow changes becomes adapted for widely different purposes”. o “This throughout nature almost every part of each living being has probably served diverse purposes and has acted in the living machinery of many ancient and distinct [species]’ Orchids have Vestigial organs ● Vestigial organs – theory of use and disuse e.g. eyes of animals that cannot see ● Before Darwin’s book, naturalists believed that these vestigial organs were specially created and arranged in their proper places by an Omnipotent hand to complete the scheme of nature. ● How are they explained? In Darwin’s day he would refer theory of use and disuse. If do not use at all, they will be lost. They had these organs as they were descended from their ancestors. But this theory has been discarded now. ● Modern biologist more Darwinian than Darwin – they use natural selection in even more areas than Darwin. ● How is it so many fish in caves, that such fish lose their eyes. Clues that they used to have eyes. They have DNA for eyes. Others have eyes that are vestigial. How can natural explanation explain them? There are benefits to lose its eyes. Eyes do not do any good in complete darkness, if a fish is occasionally born with reduced eyes, less energy is used. The Book ● In the orchid book, Darwin demonstrated that the flowers had just the right structure to guide insects into the right position to cause the pollinia to stick in this way and to receive pollen from visiting insects. ● Darwin showed that plants were just as rich with intricate adaptations as animals and that their adaptations were cobbled together from pre-existing elements by natural selection. ● Natural selection was the only scientific explanation that can explain why these things were this way. ● Darwin kicked off radical changes to study of botany through this book because structures of plants suddenly made a lot more sense than before Darwin. ● Flowering plants emerged suddenly after extinction of dinosaurs. Same with grass. ● Darwin made more prediction. ● I.e. earlier naturalists thought that the reason for all these structures was for some abstract arrangement of nature. ● Darwin said someday people will see this as absurd. Some reactions ● Most fierce critic of Darwin’s book. o Duke of Argyll’s – evolutionist creationist ● Do not like god-less version of evolution. o God guided evolution. ● A creationist’s critique sees the Duke of Argyll’s, Reign of law (1867), ● The language Darwin used of “purpose and intention” was the same as if one were talking about a creator doing the work. ● But Argyll was unconvinced by Darwin’s arguments that only natural causes have led to orchids. ● Most of the reaction was favourable. o Asa Gray:” if the Orchid-book had appeared before the ‘Origin’, the author would have been canonised (praised) rather than anathematised (condemning) by the natural theologians. ▪ Darwin replied that the Orchid book was a “flank movement” on the enemies of natural selection. o In 1874, Gray praised Darwin for finding “the explanation of all these and other extraordinary structures, as well as of the arrangement of blossoms in general, and even the very meaning and need of sexual propagation...” ● A book started out as hobby turn out to be very successful. Showed his theory can explain the details and advance his theory. ● Darwin’s later life and lesser-known works of plant ● Most people associate Darwin with evolution etc… but few know that he discovered many things about plants. Maybe because less glamorous. Darwin is mostly housebound, but his mind is still active. That is why most of his later books were more on botany. Why do some plants catch insects? ● Once during holiday, Darwin noticed little plants called Sundew. Insects got stuck in them. ● Wallace in Malacca – Why does pitcher plant have a cup? Pitcher plant always full of dead insects. ● Before Darwin 🡪 people think may be designed by God to keep number of insects down. No scientific explanation on why so many dead insects is on them. ● Darwin wanted to know why these plants have so many insects stuck on them. Convinced that it is not an accident, there must be a reason. He began to conduct experiments. Insectivorous Plants ● The first of Darwin’s three books on the behaviour of plants. ● The study results in this book. One of Darwin’s shortest title. 1875 Common Sundew (Drosera rotundiflora) Insects ● Darwin wrote about in his book 🡪 Insectivorous plants (first of the three books on behaviour of plants) ● Their structure. Modified leaves. Stalks with sticky droplets on the ends o Insect lies on it and they curl up ● Darwin’s experiments showed what substances they reacted to… nitrogen. i.e. Animals (insects) o At first, teased them only with sth but didn’t curl up, only animal substance ● How do they move? Do they have nerves? ● As evolutionist, he thinks these structures are modified leaves. ● As insect drops on it, it becomes curled up. Why does the plants curl up on the insect? ● He tried to put other things like rocks or liquid, but nothing happened. ● It turns out that the substance that the plant reacts to is not random but always animal substances. All the substances it reacts to contain a lot of nitrogen. ● Insects are attracted to them and land on them. ● After a few days it opens up again and the insect is gone ● Acidic substance like the digestive juices in stomach. The plants digest and absorb the insect but why? ● These are plants that grows only in very poor soil with very low nitrogen in swampy area. Almost the only place where these plants grow. Acquire nitrogen through the insects. How could these behaviours have evolved? ● Evolutionary history of sun dews o Absorption o Some plants produce acidic juices for other reasons o Some plants capture insects accidentally ● “If such exudation did ever occur, the solvent (digestive juices) would act on the animal matter and this would be an act of true digestion. As it cannot be doubted that this process would be of high service to plants growing in very poor soil, it would tend to be perfected through natural selection. Therefore, any ordinary plant having viscid glands, which occasionally caught insects, might thus be converted under favourable circumstances into a species capable of true digestion. It ceases, therefore, to be any great mystery how several genera of plants, in no way closely related together, have independently acquired this same power. o Species was doing this because they were living in places with very little nitrogen ● If all these put together, it will evolve to something like sundew. ● What he discovered in sundew, happen in several other plants. ● Probably evolved from hairs that trapped rain drops- on which insects were trapped and died… (speculation by Darwin ● For plants if there is water on it, it cannot respire ● Plants with beads of liquids on them probably trapped insect. Probably the source of these plants that squirt liquids which originally is just water. Venus fly trap, Dionaea muscipula – another modified leaf ● Darwin did not just study Sundew. Studied Venus’ fly trap ● Completely diff kind of plant that catch insect. Darwin: why and how did they come to do this? ● A different kind of modified leaf. ● Pitcher plant – passive. Sundew and Venus – not passive ● Darwin wanted to know, do plants have an equivalent to nerves and muscles? ● He conducted some unusual experiments… ● Darwin tried gassing them. Tried to drug his plants. If it were an animal, it will be dead. The drug did not work. ● One of the rare instances where Darwin disproved what he hopes to find. He was hoping that one of the explanations is common descent that plants and animals are from the same ancestors. Utricularia, or bladderworts ● Aquatic plants. Little bubbles – People used to think they are little floats. But they are filled with water and not air. These little bubbles have dead insects in them. ● Prey on animals like mosquito larvae ● Slightest touch (door flies open inwards, insects traps inside) ● World fastest plant killer People were shocked that plants kill & of course, as usual there were critics Movements and habits of plants (Climbing Plants), 1865 [Another journal that was later expanded to book] ● “Climbing plants may be conveniently divided into those which spirally twine round a support, those which ascend by the movement of the footstalks or tips of their leaves, and those which ascend by true tendrils. These tendrils being either modified leaves or flower-peduncles, or perhaps branches.” ● Next, he studied about how plants move. ● In particular, those that actually move but you can’t see ● Something that very slow process that you can’t watch. ● A journal 1865 article ● Ten years later publish the expanded book version ● Twining plants – twining plants make circling movements (nutation) as they grow, spiral motion. Plants no muscles or nerves, they just grow in this way. Why do they climb? Get up high without growing a big body by climbing up something else o A lot in SEA ● Some respond to stimuli: light, gravity, and touch ● Why do they climb? Because of sunlight ● Darwin made experiments. Darwin asked someone to make little wooden boxes. Put them on top of a clock to make it spin. ● Plants climb up to obtain more light. Loads of other plants already get their light and blocking others. There is competition. Climbing turns out to be cheap way of doing that. One way is by being a big tree, a tree that is really tall. But a climbing plant can grow beside it and grow the same height. ● These diagrams – Darwin’s invention for time lapse photographs for plants. Tip of plant with ink trace ● Darwin wants to know how the plant is moving. So, he invents this system to see how it moves. o Sort of growing in a spiral way. o o Marked each dot with a timestamp (how it moves over time) 🡪 like in a circular pattern ● What Darwin found; all these climbing plants are based on the modifications of circling growing up. o Mature plants wont have leaves on the ground but nearer to the top (for sunlight) Cross and self-fertilisation ● In this book, Darwin makes his point about a strong bias in nature for cross fertilisation. ● If you make illegitimate crossing, Seedlings of self-fertilised parentage are inferior. They are weaker and less numerous. ● He was convinced of benefit of cross fertilisation. ● Cross pollination provides variability and natural selection adapts 🡪 what structures helps cross pollination? [SECOND BOOK] The different forms of flowers, Plants of the same species, 1877 ● What structure helps pollination? ● Flowers on different plant with styles (receive pollen) and stamens (produce pollen) of two or three different lengths o These ensures cross pollination and prevents self-pollination o Different forms of flowers is called heterostyly ● In the diagram of a primrose Darwin shows how a dimorphic plant’s construction increases the chances for cross-pollination. A insect visits the flower on the right with long stamen, exits dusted with pollen. It then flies to flower on the left with a long style. Pollen on that insect is perfectly positioned to rub off onto the flower’s stigma. o Like the barnacles (two seces) ● Heterostyled flowers – differently (hetero) shaped flowers. Some plants even have 2-3 flower types. o These flowers play diff roles to ensure cross pollination ● Linnaeus use different forms of flowers to classify species. ● 2 diff kinds of flowers in this plant, called heterostyly. ● Primula Vulagris, Long-styled form and short-styled form, both are perfect hermaphrodites but bear same relation to one another as do the two sexes of an ordinary animal. ● I.e. if you are a plant and you want to make sure you get a pollen from another plant of another species. By having a flower type A, you can be sure you are only pollinated by another plant type B. ● If the insect visits the same plant/flower twice, nothing will happen, cannot fertilise itself ● A complicated way to ensure cross pollination Lythrum salicaria (purple loosestrife) ● Confusing because 1 species with 3 different flower types, Lythrum salicaria (purple loosestrife) ● To ensure cross pollination, long from plant A to long from plant B ● 3 species / types / forms 🡪 6 combinations of pollination process (of two distinct species) o Prevent same pollination o Helps cross pollination [THIRD BOOK] The power of movements of plants, 1880, wrote with his son, Francis Darwin ● Depends on sensitivity to light and gravity. Not all grew by revolving, some bent side to side as they grew (not circular fashion) o Shows how light is passed through the plant as the plant lean towards the light (when they grow) and not just land on the light ● The plant sleeping. ● More upright during the day. At night ‘sleeping’ to protect themselves from the cold. ● All these different types of plant movements, e.g. of Darwin applying his theories to all parts of nature. Similar rules applied to new cases. Examples of old structures had evolved into doing something new. ● Cartoon drew by Linley Sambourne drew Darwin as an ape on a tree, “Darwin’s movements and habits of climbing plants” o Making fun of Darwin (as per usual) He has a tendency to exaggerate. Like “this is the most destructive objection of my theory…” ● “in accordance with the principles of evolution it was impossible to account for climbing plants having been developed in so many widely different groups, unless all kinds of plants possess some slight power of movement of an analogous kind. This I proved to be the case, and I was further led to a rather wide generalisation, that the great and important classes of movements, excited by light, the attraction of gravity, are all modified forms of the fundamental movement of circumnutating. It has always pleased me to exalt plants in the scale of organised beings; and I therefore felt a special pleasure in showing how many and what admirably well adapted movements the tip of a root possesses.” ● Darwin, Autobiography. ● Darwin was also a botanist o Adaptations that we see must benefit the organism. (Natural selection) ▪ Natural selection change sth that existed before o Modification of something that existed before, change function