The Life of a Tree: 3. Growth

Most animals are integrated in another important way: their growth.  You (as a representative animal) grew from the cell that resulted from conception in a highly organized dance in which cells actually moved from place to place, divided (at different rates in different places and times), and differentiated into all the many tissues and organs that make up the adult human.  (Just how all of this is organized and regulated is cutting-edge science today.)  Then, once you had reached adult size, your cells stopped dividing.  Just stopped.  --with the exception, of course, of those needed to replace skin, blood cells and the like, and those triggered to divide in wound healing, and so on.  

Plants do this is a very different and simpler way.  Plant cells are encased in a cell wall that strengthens them, provides overall structure to the plant, and effectively fixes the individual cells in place--and is one reason plants don't move.  Instead of growing everywhere, land plants grow at shoot and root tips, at regions called meristems.  Cells in a meristems divide, then those that end up on the side opposite the tip differentiate into vascular tissue, fibrous tissue, cortex "filler," leaf primordia, etc., while those that are nearer the tip continue to divide, gradually leaving the differentiated stem or root cells behind as the shoot elongates.  

 Shoot (Coleus?) and root (onion) meristems.  Tissue slices have 

been stained to show individual cells and their nuclei more clearly. 

One consequence of apical growth that surprises people is that the tree branch you have to duck under this year will never be any higher: its height was established when that branch was only a twig.  (Those lower branches tend to die over time, though, often leaving the lower trunk bare.)   

Unlike animals, most plants have no "adult size": as long as they live, trees and shrubs continue to grow.  Because each year's shoot growth begins with last year's buds, to cease to grow is death.

Besides growing at tips, woody plants (trees, shrubs and vines) also grow around their circumference.  A layer of tissue under the bark, a lateral meristem called vascular cambium, adds cells inward to form water-carrying xylem tissue, and outward to form sugar-carrying phloem tissue.  (When you look at a piece of wood, you are looking at xylem, and the tiny holes sometimes visible in end grain are the cut ends of water-carrying tubes called xylem elements.)  While xylem is long-lasting, typically carrying water for many years before finally clogging up and becoming dark-colored "heartwood", phloem is only active for a short period, eventually becoming a second kind of lateral meristem: bark cambium.

This is a good place to talk about the chemistry of animal and plant strength.  We mobile animals are a peculiar mixture of delicacy and strength: our individual cells are floppy, insubstantial little blobs of Jello, but together they secrete proteins that form immensely strong fibers (mostly collagen) that make our bodies tough and strong and yet flexible.  The walls of plant cells are made of a very different fiber called cellulose, which is a polymer of sugar molecules instead of a protein.  

 Loose cells are easily scraped off the inside of your cheek with a dull toothpick.  

These are stained to show the nuclei.  Notice how floppy they are!

Cells in the leaf tissue above have been soaked in a salt solution so they have wilted.

The living tissue of each cell (complete with green, disc-shaped, sugar-generating chloroplasts)

have collapsed, leaving the box-like cell walls intact.  Think of severely wilted lettuce.

 A cross-section through a stem shows thinner-walled cortex cells with thicker-walled cells.  Producing linen begins with beating stems of flax to separate these strong vascular fibers

from the rest and spinning them into thread. 

The little cellulose boxes in which plant cells live make them individually tough even as they trap the cells in place.  (In fact, green plant cells are hydraulic structures: their rigidity is due to internal water pressure in exactly the same way a football's rigidity is due to air pressure.)*  Animals must continually bathe their cells in a mild salt solution that prevents them from either shriveling up (too much salt) or exploding (too little).  This vulnerability is the reason athletes must watch their electrolyte (salt) balance.  Though plant cells may wilt with too little water (or too much salt) , these cells are immune from damage by fresh water because their cell walls are strong enough to prevent their bursting.  Together, these cell walls form the fiber of countless natural products, from the cotton in our clothes, to the rope that formed the rigging of tall ships.  In a form stiffened by other molecules (lignin prominent among them), this fiber becomes the wood that builds our homes.  Wood, sometimes disparaged in comparison with modern materials, remains stronger for its weight than any other substance. 

*I remember the first time I handled a deflated football: the pigskin is soft and flexible (like a plant cell wall).  Inside it is a delicate but air-tight rubber bladder (like a cell membrane).  The pigskin prevents the inflated bladder bursting just the way a cell wall prevents the plant cell membrane from bursting.  

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!

The Life of a Tree: 1. Integration--and the lack of it

Humans are very much animals.  We think like animals, and we have the most regard for the other animals that are our closer relatives.  Plants are so different from us that they're hard to understand.  That very foreignness may be one reason I like them. 

Let's contrast them. 

Yes, insects like these milkweed tussock moth caterpillars are animals.

(Ask yourself: does it eat? then it's likely an animal.  Most also move.)

Most animals move.  Animals have sophisticated senses: chemical sensors (the nose), light and image sensors (eyes), vibration sensors (ears), etc.  Animals--at least, those much more complicated than jellies, sponges, flatworms, and clams--have nervous systems that provide very sophisticated and centralized coordination and control.  They circulate blood in a loop-like circulatory system, they have nerves with a brain that receives and processes information and responds back.  A sensory stimulus to any part of the body might result in a response by any other part or by the entire body.

Ponkapoag Pond, Blue Hills Reservation, August 2016.

Plants have none of this.  They do not generally move, and need not seek food since they make their own using light energy; indeed, photosynthesis is one of the characteristic traits of the plant kingdom.  They have two more-or-less independent one-way transport systems: one (called xylem) transports water and nutrients from the roots toward the stems and leaves where they are important to photosynthesis; another (called phloem) transports the food (sugars and the like) made in photosynthesis to tissues that cannot make their own, such as the stem and roots. 

Plants respond to stimuli, also.  They can sense gravity: roots grow reliably down, shoots upward.  Plants are sensitive to light, and in particular use the length of darkness to help regulate seasonal changes.  Temperature is a stimulus that, for example, triggers the beginning of spring growth.  Plants are sensitive to various chemicals, and can (famously) respond to the chemicals produced by nearby plants under insect attack: they will increase their own production of defensive toxic chemicals.  But there is no nervous system and very little centralization; responses are decentralized and local. 

A tree I remember on my college campus had grown up and enveloped a streetlight; as fall lengthened into winter, all the leaves turned and fell except those in a sphere around the light: these remained green as they became increasingly tattered and finally died in the winter without turning--all because those leaves did not experience the long darkness that signals fall to a tree.

Trees seem to behave in coordinated ways in sending roots out into more moist or fertile soils, or increasing root or shoot growth in a way that balances the two.  But these are very simple and mechanistic responses: roots in richer soil have more resources and so will grow faster, while a cut tree stump will often sprout rapidly because it has all the resources of a disproportionately large root system supplying water and nutrients.  The result only looks purposeful. 

Plant vascular tissue.  Xylem--a dead tissue that resembles household plumbing

--transports water and minerals from roots to leaves, drawn upward by evaporation of water

from leaf cell surfaces.  Phloem, made of living cells, transports sugar by bulk flow

from where it is most concentrated (in summer, the leaves where it is created) to where it is

least concentrated (stems and roots that need food but cannot make it as green leaves can).