The evolution of seeds by the seed plants, including the gymnosperms, facilitated their invasion of land. Encased in a seed, the embryo of seed plants can withstand the extremes of the terrestrial environment such as freezing weather, drought, and even fire. Other characteristics, such as a thick cuticle on the epidermis, sunken stomata, and a hypodermis, consisting of one to several layers of thick-walled cells, also enable gymnosperms to withstand harsh environmental conditions from the freezing winters of the far north to dry desert areas. Resins produced by gymnosperms provide defense against fungal and insect attacks, while mycorrhizal fungi increase availability of soil nutrients and water to gym-nosperms. Mutualistic associations with squirrels and birds, such as nutcrackers, promote gym-nosperm seed dispersal. Gymnosperms provide humans with a wealth of "nature's services," ranging from construction materials to cancer treatments.
resin canals cork xylem ray phloem vascular cambium annual ring of xylem pith
Figure 22.3 A portion of a cross section of a pine stem, showing annual rings.
lost a few at a time so that some functional leaves are always present on a healthy tree.
Wood varies considerably in hardness. Most gym-nosperm wood, including that of pines, consists primarily of tracheids and differs from the wood of dicots in having no vessel members or fibers. Conifer wood, because of the absence of thick-walled cells, is said to be soft, while the wood of broadleaf trees is described as hard.
In many conifers, the annual rings of the xylem are often fairly wide as a result of a comparatively rapid growth rate during the growing season. Resin canals are formed both vertically and horizontally throughout various tissues (Fig. 22.3). The bark includes the secondary phloem and may be relatively thick. It often becomes 7.5 centimeters (3 inches) or more wide, and in the giant redwood, it may become as much as 60 centimeters (2 feet) wide. Companion cells are absent from the phloem, but similar albuminous cells apparently perform the same function.
Mycorrhizal fungi are associated with the roots of most conifers. In fact, pine seedlings that germinate in sterilized soil do not grow well at all until the fungi are introduced or allowed to develop. The roots of adjacent pines often interweave. New England pioneers made use of this characteristic in eastern white pines when clearing land. After trees were felled, they would tip over the stumps with the roots still attached and use them for fences. The fences survived for many years.
arranged in a spiral or in whorls around an axis; they are usually produced in the spring. The pollen cones usually develop toward the tips of the lower branches in clusters of up to 50 or more (Fig. 22.4) and are mostly less than 4 centimeters
Introduction to See d Plants: Gymnosperms 425
(about 1.5 inches) long. Microsporangia develop in pairs toward the bases of the scales.
Microsporocytes in the microsporangia each undergo meiosis, producing four haploid microspores. These then develop into pollen grains that each consist of four cells and a pair of external air sacs. The air sacs look something like tiny water wings (Fig. 22.5) and give the pollen grains added buoyancy that may result in the grains being carried great distances by the wind.
Pines produce pollen grains in astronomical numbers. For example, it has been estimated that each of the 50 or more pollen cones commonly found in a single cluster may produce more than 1 million grains, and there may be hundreds of such clusters on one tree. The grains accumulate as a fine yellow dust on cars, shrubbery, or anything else in the vicinity, and they often form an obvious scum on pools and puddles. Within a few weeks after the pollen has been released, the now shriveled pollen cones fall from the trees.
Megaspores are produced in megasporangia located within ovules at the bases of the seed cone scales. The seed cones (female strobili) are much larger than the pollen cones, becoming as much as 60 centimeters (2 feet) long in sugar pines and weighing as much as 2.3 kilograms (5 pounds) in Coulter pines. When mature, they have woody scales, with inconspicuous bracts between them, arranged in a spiral around an axis. They are mostly produced on the upper branches of the same tree on which the pollen cones appear (Fig. 22.6).
Introduction to See d Plants: Gymnosperms 425
Figure 22.5 Pollen grains of a pine, as seen with the aid of a microscope. Each pollen grain has a pair of air sacs that provide added buoyancy, x300.
The ovules (Fig. 22.7) occur in pairs toward the base of each scale of the immature seed cones and are larger and more complex than the microsporangia of pollen cones. Each ovule has within it a megasporangium containing the nucellus and a single megasporocyte. This, in turn, is surrounded and enclosed by a thick, layered integument. The integument has a somewhat tubular channel or pore called a micropyle that is pointed toward the cone's central axis. One of the integument layers later becomes the seed coat of the seed.
A single megasporocyte within the megasporangium of each ovule undergoes meiosis, producing a row of four relatively large megaspores. Three of the megaspores soon degenerate. Over a period of months, the remaining one slowly develops into a female gametophyte that ultimately may consist of several thousand cells. The nucellus is used as the food source for the growing gametophyte
As gametophyte development nears completion, two to six archegonia differentiate at the end facing the micropyle. Each archegonium contains a single large egg. When a stained, thin, lengthwise section of a pine ovule is examined with a microscope, only one archegonium or egg may be seen, or the micropyle may appear to be missing. This is due to the section not having been sliced precisely through the middle of all the structures.
Seed cones are at first usually reddish or purplish in color and commonly take two seasons to mature into the green and finally the brownish woody structures with which we are all familiar (Fig. 22.8). During the first spring, the micropyle pollen chamber integument (2n)
nucellus archegonium egg female gametophyte (n)
Figure 22.7 A longitudinal section through part of a pine ovule, x100.
immature cone scales spread apart, and pollen grains carried by the wind sift down between the scales. There they catch in sticky drops of fluid (pollen drops) oozing out of the micropyles. As the fluid evaporates, the pollen is drawn down through the micropyle to the top of the nucellus.
After pollination, the scales grow together and close, protecting the developing ovule. Meiosis and megaspore development don't occur until about a month after pollination. After a functional megaspore is produced, the female gametophyte and its archegonia don't mature until more than a year later.
Meanwhile, the pollen grain (immature male gameto-phyte) produces a pollen tube that slowly grows and digests its way through the nucellus to the area where the archego-nia develop. While the pollen tube is growing, two of the original four cells in the pollen grain enter it. One of these, called the generative cell, divides and forms two more cells, called the sterile cell and the spermatogenous cell. The spermatogenous cell divides again, producing two male gametes, or sperm. The sperms have no flagella (unlike the sperms of other organisms discussed in the preceding chapters) and are confined to the pollen tube until just before fertilization occurs. The germinated pollen grain, with its pollen tube and two sperms, constitutes the mature male gametophyte. Notice that no antheridium has been formed.
About 15 months after pollination, the tip of the pollen tube arrives at an archegonium, unites with it, and discharges the contents. One sperm unites with the egg, forming a zygote. The other sperm and remaining cells of the pollen grain degenerate. The sperms of other pollen grains present may unite with the eggs of other archegonia, and each zygote begins to develop into an embryo that is nourished by the female gametophyte. This is similar to the development of fraternal twins or triplets in animals. At a later stage, an embryo may divide in such a way as to produce the equivalent of identical twins or quadruplets in animals. Normally, however, only one embryo completes development. While this development is occurring, one of the layers of the integument hardens, becoming a seed coat. A thin membranous layer of the cone scale becomes a "wing" on each seed. The wing may aid in the seed's dispersal. Squirrels and other animals may also help dispersal by breaking open the cones.
In other species, such as the lodgepole, jack, and knob-cone pines, the cones remain on the tree with the scales closed until they are seared by fire or open with old age. Sometimes, these cones are slowly buried as the cambium adds tissues that increase the girth of the branch. Seeds of such engulfed cones have been reported to germinate after they have been dug out of the stem.
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