Saturday, November 26, 2016

The Life of a Tree: 2. How do Plants Have Sex?

--Continuation of an essay comparing plants to animals.  (Part 1 is below.)

Yes, insects are animals.
(If it eats, then it's likely an animal.  Most also move.  None make their own food.)
Here are two damselflies mating; sexual reproduction is nearly universal 
among life forms more complicated than bacteria. 

Plants reproduce sexually in fundamentally the same way most animals do: they produce sperm and eggs with half the usual chromosome number, then sperm from one individual meets egg from another, and they fuse to form a new individual with genes from both parents.  But there are differences.  

First, all plants "alternate generations": they form individuals with half chromosome numbers (called haploid) as well as full chromosome numbers (diploid).  This is most striking in mosses and liverworts, where these individuals may live independent lives.  It would be as if your eggs or sperm went off on their own, grew multicellular bodies, and lived independent lives before producing sex cells that finally resulted in a baby!  In "higher" plants such as conifers and flowering plants, the haploid individuals are very small and incapable of living long without dependence on the more familiar adult, so that only botanists notice them.  (A pollen grain is actually such a haploid individual: it is made up of three cells wrapped in a tough, water-resistant shell that enables it to survive until it reaches another flower.)

Alternation of generations in the life cycle of ferns.  The diploid "sporophyte" is the familiar fern
we see.  The haploid "gametophyte" lives underground and resembles (I'm told) a wad of chewing gum: after fertilization (fusion of egg & sperm) the gametophyte grows upward from it.

Second, transportation is an issue.  For most animals, sperm get to eggs either by swimming through the water in which both parents live or, in the case of land animals, are brought into close proximity by mating.  Land plants face the same challenging waterless environment as land animals, but with the handicap of being unable to move.  Land plants of moister places, such as mosses and liverworts and ferns, employ a swimming sperm strategy just as some marine animals do.  Other land plants overcome the challenge of immobility by producing pollen to carry the sperm from male to female.  Those that produce pollen include conifers and flowering plants.  Most such plants have evolved one of two solutions to transporting their pollen: allowing dusty pollen grains to waft on the wind, or making a business arrangement with an animal that can do the transport.  

Wind pollination works pretty well where individual plants are in close proximity, such a field of grass.  Many trees also are wind pollinated.  (Many people are unaware that grass and most trees are flowering plants: since they do not need to attract animals, their flowers are usually small and not brightly-colored.)  These are the plants that might make you allergies act up. 

 Pollen grains can be quite beautiful.  The shapes are specific enough  that palynologists studying pollen grains in old lake sediments can name the plants that grew nearby thousands of years ago.
(Spiky projections help some stick to insects, etc.)

For those plants that engage the services of animals to transport their pollen, insects--and especially bees--are the most common.  (So many crop plants are pollinated by honey bees, for example, that the decline in honey bee populations caused by Colony Collapse Disorder actually threatens the American food supply.)  Other animals that have partnered with flowering plants include wasps, moths and butterflies, bats, and birds.  The coevolution of plants and pollinators has led to business partnerships in which the plant offers a reward (often nectar) to lure the animal, then dusts it with pollen, and allows it to go off to visit other flowers.  Sometimes these arrangements have become so specific that a single species of plant depends on a single species of animal, which in turn has become wholly dependent on that plant.; this is highly-efficient for both species, but also very risky in the long run, should one or the other go extinct.  Orchids are famous for pulling a rather dirty trick: growing flowers that imitate particular insects, luring the males for mating--then sending  the poor, disappointed creatures to go elsewhere with their pollen load--likely to another orchid to repeat the attemp! 

Third and arguably strangest for us, as animals, to consider is the matter of gender.  Although gender is changeable in some animals (slipper shells come to mind), we mostly think of our fellow animals as either male or female.  But gender varies in plants.  Most flowering plants are simultaneously male and female: you can find dusty, male pollen-producing stamens and also sticky, female, pollen-receiving stigmas on the same plant, and most often even in the same flower.  Rarer is the case of plants that produce only male or female flowers.  Around here, the "gendered" trees include ashes, quaking aspen, and red maple and ash-leaved maple.  Though most flowering plants produce both eggs (which remain inside the flower) and sperm (in pollen),  they do not readily pollinate themselves: chemical recognition seems to make self-pollination rare in most species.  This is probably because in-breeding is as bad in the long run for plants as it is for animals.  

Stamens are the male parts; the anther makes and releases the pollen.  The pistil (carpel) is the female part; pollen lands on the stigma, grows a tube down the style into the ovary and to the egg inside the ovule, and the sperm move down the tube to fertilize the egg.  (The names of many parts were borrowed from animal anatomy, but only roughly correspond.)

The plan of individual flowers varies widely, also.  Flowers which are perfect (both male and female) and complete (having all the typical parts such as sepals and petals) are common.  But all possible variations are found.  Flowers of grasses, for example, have no petals (having no need to attract pollinators), while many (lilies and tulips, for example) have sepals that have become so like the petals that they are collectively known as tepals.  The Norway maple, a common city tree, can have--growing in a single bunch--perfect flowers, male flowers and female flowers!

No comments:

Post a Comment