How do plants breathe?

… wait… did you say plants breathe?

Most folks have learned that plants take up carbon dioxide from the air (to be used in photosynthesis) and produce oxygen (as a by-product of that process), but less well known is that plants also need oxygen. Plants, like animals, have active metabolisms, fueling all bodily activities. For this, almost all organisms need oxygen (a few use sulfur instead), which interacts with glucose (from the breakdown of organic compounds) to produce energy, and the complex process produces carbon dioxide (and water molecules) as a by-product. Most of the carbon dioxide is used by the plant for photosynthesis, but any excess needs to be eliminated. So plants need to breathe—to exchange these gases between the outside and the inside of the organism. Breathing is part of a long, complex process called respiration, much of which occurs inside cells, where the metabolic machinery produces energy.

Although many land plants get some of their oxygen from water that rises from the soil through conducting tissues (the water-conducting tissue is called xylem, which forms the wood in trees), water doesn’t supply enough—plants also need to take in oxygen from the air. This is not a simple matter, because the outer coverings of plants are impervious to the passage of water, protecting them from desiccation. But these coverings also prevent the passage of carbon dioxide and oxygen.

However, evolution solved this dilemma by incorporating a sort of ventilation system into the plants’ aerial exteriors. Many tiny pores (called stomata) are scattered densely over the surface of leaves and stems; they are especially dense on leaves, sometimes thousands per square inch. Movement of gases in and out of these minute pores is regulated by two special cells on each side of the opening; these cells can enlarge, to close the pore, or shrink, to open it. (Stomata also help regulate water uptake and loss, but that’s another topic…). Oxygen entering the stomata diffuses to areas of lower oxygen concentration inside the plant. Eventually it reaches the metabolic machinery in the cells, where it is used to oxidize glucose and produce energy.

One day, I happened to notice some alder branches with numerous lumps on the bark. These are called lenticels, which are pores that open through the bark into the underlying wood. They too allow the passage of gases between the outside air and the living tissues of the wood. Lenticels often have raised, stiffened edges, which are thought to prevent excessive deformation as the plant grows and the stems thicken (which makes me wonder if the very lumpy alder branch that prompted all this verbiage had been under lots of stress while growing). In the central part of a lenticel the cells are widely spaced, leaving room for the passage of gases by diffusion.

We see lenticels on lots of trees. For instance, on birches, they are conspicuous as horizontal, black marks on the white bark. On alders, they seem to take various forms, with or without the very lumpy ridges of bark around the opening. On some trees, they are sometimes quite hidden under thick bark layers but may appear at the bottom of crevices in the bark.

It turns out that many land plants have lenticels, not just on the woody parts. For instance, apples and pears have lenticels on the fruit skin. Sometimes these openings become enlarged and discolored, affecting the visual attractiveness of the fruit. On potatoes, when the soil is too wet, the lenticels enlarge, to admit more oxygen, and become unsightly—if they then dry, they look like scabs all over the tuber. Grapes have them both on the fruit and on the little stem that attaches each grape to the plant. A recent study caused much excitement among grape-growers and wine-makers, because the study revealed that oxygen deprivation, when the lenticels are blocked, caused increased cell death in the grapes, so that they did not contain the normal amount of water, changing the flavor. Oxygen uptake is affected by temperature, and at least in some varieties of grapes, cell death increases with temperature. So there is concern among grape-growers, wine-makers, and wine connoisseurs that climate-warming would increase oxygen deprivation and increase cell death in grapes.


some tidbits about a lesser-known local tree

On a nice winter day, a cluster of Parks and Rec hikers perched for lunch at the edge of a beautiful muskeg. Someone observed that many of the shore pines were rather stunted and often crooked, while others grew straight and tall. Our local shore pines are a distinct subspecies of lodgepole pine, which grows mainly in the Interior; its straight, tall growth form gave the species its common name. Some of these Interior-type lodgepoles are reported from the north end of Lynn Canal.

The question, that day, was whether or not the tall specimens in our muskeg might be strays from up north. Someone remembered that there are subtle differences between the subspecies in the orientation of the cones on the branches. But without a detailed genetic analysis, this notion probably cannot be ruled out—after all, these pines use the wind to disperse both pollen and seeds, and who’s to say that no genes from the upper Lynn Canal population have ever come to Juneau.

On the other hand, we observed that the tall, straight pines in our muskeg grew chiefly along the edges, near the surrounding spruce and hemlock forest. This distribution suggested to us that maybe the growth form is determined by habitat; for instance, muskeg edges tend to be dryer than the main part, and perhaps the acidity is somewhat less, too. A little research, back at home, revealed that expert plant ecologists have come to the same conclusion.

The “contorta” subspecies. Photo by Kathy Hocker

That little discussion reminded me that pines are interesting in several ways. I have room to deal with one of them here.

About forty species of pine occur in North America (out of over 100, worldwide). Most of these produce seeds with well-developed, flat wings; they are adapted for wind-dispersal when the cones open and shed the seeds. Just a few kinds lack wings altogether or have extremely small wings.

The best-known North American species with wingless seeds are the several closely-related species of pinyon pine. They grow on poor soils in dry areas of southwestern U.S. and Mexico. The seeds are large, over a centimeter long, and well-endowed with highly nutritious endosperm (about 60% fat) to fuel the growth of seedlings. When the seeds are mature, the cones open but hold the seeds on the cone scales, not releasing them to just fall to the ground. This is considered to be an adaptation for seed dispersal by birds, principally pinyon jays and Clark’s nutcracker, although scrub jays and others also participate. The birds harvest ripe seeds from the handily open cones and commonly cache them all over the landscape, sometimes may kilometers away from the parent tree. Many of the cached seeds are retrieved and eaten by these birds with excellent memories, but some are lost—and these can produce new trees for another generation. Squirrels don’t miss these tasty bites, of course, but they are mainly seed predators. Any fallen seeds—and some cached ones—are scarfed up by rodents, quail, and other ground foragers, including humans.

Pinyon jays and Clark’s nutcrackers both have very strong bills, for hacking open closed, green cones early in the season, before the cones open. Both species have special anatomical adaptations for carrying loads of seeds to caches. The upper esophagus of the jays expands when it is packed with seeds; as many as forty pine seeds can be carried at one time. Clark’s nutcracker has a pouch under the tongue where dozens of seeds can be carried. Both of these birds eat other kinds of seeds too, as well as insects and other foods, but the relationship between the birds and the pinyons is considered to mutualistic—with benefits to both sides. Less specialized birds, such as scrub jays, participate in the mutualism, but the relationship is less specialized.

Two additional pine species (limber pine, southwestern white pine) in western North America make seeds with vanishingly small wings; their seeds also dispersed by caching birds. Seeds are released from the cones when mature and are available then to ground foragers. The seeds of the southwestern white pine are eagerly harvested and cached by Mexican jays, which cannot open green cones well and apparently eat more acorns than pine seeds.

One more North American species (whitebark pine) makes wingless seeds. But in this species the cones do not open readily. This species grows in montane forests of the western U. S. and British Columbia, where seed-harvesting birds and squirrels generally have to open the cones to extract the seeds. The caches of these harvesters are regularly raided by black and grizzly bears. At least in some areas and some years, this food source contributes significantly to the survival and reproductive success of the bears.

Wingless pine seeds also occur in Eurasia, where winglessness seems to have evolved independently several times. The spotted nutcracker there participates in a mutualism similar to that in North America., but the Eurasian jay seems to be more closely associated with acorns and beechnuts (as is the blue jay of eastern North America). Who else might participate in a mutualism with wingless-seeded pines in Eurasia?

Another question: What are the historical and ecological factors that determined the lack of wingless pine seeds in Southeast, where the seeds of all conifers are basically dispersed by wind?

Spring medley

progress of a favorite season

Spring is officially here: the vernal equinox has gone by and the days are rapidly lengthening. There are much livelier signs of spring as well. Sapsuckers have arrived in force, rat-atat-tating on rain gutters and stove pipes (and trees). Juncos trill at the forest edge and song sparrows are tuning up in the brush above the beaches. Pacific wrens sound off from invisible lookouts in the understory. Best of all, ruby-crowned kinglets can be heard, high in the conifers, calling ‘peter-peter-peter’ or singing their full, cheerful song. That’s when spring is really here, for me.

A walk on a favorite beach on Douglas Island was focused on finding mermaids’ purses—the egg cases of long-nosed skates. Every year, about this time, we find them washed up in the wrack at the high tide line—there must be a nursery just offshore. On this day, we found sixteen eggs cases, mostly black, dry, and in various stages of decrepitude. Just a few were still mostly whole and khaki-colored, and two had natural openings at one end, where perhaps the young skate had exited. All the egg cases had sizable holes punched into them. I would love to know if marine predators had nabbed the developing embryos or if the holes were made by a tardy, would-be predator just hoping that an embryo was still inside.

A good find in the rolled mats of rockweed at the high tide line was the body of a sea star, entirely eviscerated. All the gonads and digestive parts had been cleanly removed, neatly exposing the calcareous skeleton of the water-vascular system that runs from the center of the star out into each arm. In a living sea star, the canals of this hydraulic system are filled with fluid, mostly sea water. Numerous branches of the main canal lead to the tube feet (often visible in a live star, in rows under each arm) that function in locomotion and in opening clams. When the tube feet are extended, their ends stick to the rocks or the clam shell, and muscles in the feet contract, pulling the animal forward or pulling the clam shell open. We sometimes see a sea star humped up over a partly open clam while the star is having dinner.

A stroll on the Boy Scout/Crow Point trail led to the goose-flat covered with hundreds of crows fossicking in the dead, brown vegetation. Lots of searching and probing. Sometimes half a dozen crows would suddenly converge on another one, everybody poking at something. Apparently, successful hunts were not very common and the gang thought that sharing was appropriate.

Lots of Canada geese were scattered in small groups on the flats, in the river, and in the vegetation by the river. There were mostly head-down, intent on foraging—grubbing for roots and such, and of course talking to each other. Occasionally, two of them would take off and wing around in a wide circle before landing back where they started. One of these duos took off upstream—perhaps a mated pair about to look for a nest site in the forest.

As we often do, out there, we encountered a fellow we call the Raven Man, who carried a big bag of dog biscuits to feed the ravens. He does this from time to time, and the local ravens recognize him. As he passes through each raven territory, the residents come to greet him and cadge some biscuits. We watched some of these ravens carry five biscuits at a time, first stacking them up in a neat pile so they could be held in the bill. A dog, with some hikers, came along later and sniffed out places where ravens had cached their loot, covering it with grass or moss—surprising the hikers who were not expecting to see dog biscuits in the moss.

Most folks in Juneau are glad to see the snow disappear, at least at the lower elevations. But I loved the good snows we had in February, and here are a few flash-back memories.

–Weasels had been very active in the Peterson Creek meadows and Amalga meadows. They bounded over the clean snow, ranging widely. Every so often, the trail dove straight down under the snow and re-appeared a few feet beyond or disappeared under the overhanging edge of a frozen slough. I think they were hunting voles, whose tunnels run under the snow; did they dive down in response to the sound or fresh smell of vole or were the dives just exploratory? Another treat in one meadow were well-defined trails of mice, showing a good tail-drag.

–On the west side of Mendenhall Lake, one day I found a set of tracks running way out onto the snowy ice and right back again. It was clearly a member of the weasel family, probably a mink. What was it doing??

–A snowshoe trek up a creek out the road was a bonanza of tracks (and no recent human tracks). In the woods on the way up the hill, there were tracks of deer, mouse, weasel, squirrel, and maybe a marten. Big excitement of some large tracks that were surely those of a wolverine—the toes and the gait gave it away. The most fun was seeing a set of wolf tracks coursing over a frozen pond that sparkled with sun-struck hoarfrost.

Now the fun in the snow is finished for the year, and the fun of spring begins. Juneau folks typically love to note the progress of spring, as the season unfolds. Skunk cabbage emerging, pussy willows appearing, blueberry buds expanding, the gradual arrival of more kinds of birds, ravens carrying sticks for a nest—they all mark the progress of a favorite season.

Spring colors

glimpses of red, yellow and purple herald the season

There often comes a time in early spring when the seasonal progress seems to stall—there are still freezing temperatures at night, many ponds are still ice-covered, the iris shoots in the meadows aren’t getting perceptibly bigger, meadow grasses and sedges lie flat and dead, the lady ferns stay humped under their old dark fronds—and we get impatient for more signs of spring.

That is a good time to notice little spots of color in the forests and meadows. Folks who live in Southeast had better like green and gray, because those colors are the common background on the landscape—green conifers and frequent gray clouds. One can add ‘brown’ for all the dead grasses and sedges lying in the meadows. But the little bright spots of other colors are a visual treat, adding interest to a walk.

Touches of red pop up in several ways:

–Ruby red berries of so-called false lily of the valley lie nestled like glowing jewels in the moss. These are last year’s fruits that typically don’t ripen until they have overwintered. They will feed the early-arriving robins and then the hermit thrushes.

–Red twigs of the early-blueberry shrubs gleam, adding a pleasing contrast in the still-leafless understory. That observation brings up a question: why do these twigs turn red but not (or so I am told by those who know more than I do) those of the later-blooming Alaska blueberry?

–A few translucent red berries of high-bush cranberry hang at the ends of thin branches, uneaten by bears or pine grosbeaks or anybody else last fall or winter.

–A flash of red on the side of a tree trunk helps to advertise the presence of a red-breasted sapsucker as it hitches its way upward, tapping the bark.

–Along the roadsides, the male catkins of red alder make a swathe of a duller red that is nevertheless very noticeable against the conifers’ green. As the catkins mature, they droop and gradually open to release pollen, and the redness fades.

In residential areas, gardens of multicolored crocuses attract queen bumblebees, busily searching for nectar deep in the flower and grooming pollen off their heads. Some of them probably collect pollen too. Away from settled areas, however, those queens have only male pussy-willows as a source of nectar or pollen, until the early blueberries bloom (adding pinkish-white to the color-scape).

A favorite of many folks is the bright yellow of skunk cabbage. First appearing as a yellow spear emerging from wet places, the hood (or spathe) around the cylindrical inflorescence expands. It helps attract pollinating insects and also happens to provide shelter for the little beetles that come to the small flowers of the inflorescences to feed—and also to court and mate, and incidentally pollinate the flowers. Skunk cabbage provides a ‘progressive party’ of color, because different stands mature at different times as their specific locations warm up. Even one skunk cabbage is delightful; a whole pond full of them is spectacular.

Many of us look for purple mountain saxifrage in early spring. It likes to grow on cliffs and other rocky places, so it is very localized. We always feel rewarded when we find the first blooming ones each spring.

At somewhat higher elevations, Cooley’s false buttercups make splashes of yellow. And don’t forget to look for the violets!

Of course, impatience doesn’t suffice to hurry spring along. But it will come—flocks of robins now skitter along beaches, mallards congregate in the ice-free part of Riverside Park pond, the early songsters are heard more often. Ruby-crowned kinglets now serenade my house daily!

I never tire of watching the prolonged arrival of spring. The basic patterns are generally consistent, but always with some little variations and even surprises. This year, the big difference is what is missing: there is very little snow on the mountains. The rocky mountainsides are showing all too clearly and the usual cornice on Thunder Mountain hardly developed at all. The lack of snow ‘upstairs’ will surely have serious consequences, reducing our sources of water and hydropower and the water supply for the creeks where salmon usually spawn.