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35 Chapter 3 Reproduction of Conifers Pollen Cones and Pollen An ovule is a seed before it is fertilized by pollen; only after fertilization does the ovule develop into a seed that matures until it is capable of germination. So far we have considered only seed cones. Now we come to pollen cones and pollen. Pollen cones (figures 3.1 and 3.2) are smaller than seed cones and do not persist for nearly as long. They shrivel and dry up as soon as the pollen has been shed in spring. Although most of the pollen cones soon fall off the trees, dried up, dark brown pollen cones can still be found on the twigs of pine trees right into late fall or winter; they fall off at a touch. In the great majority of conifer species, seed cones and pollen cones grow on the same tree. Yews and junipers, however, have separate female and male trees: the females bear only seed cones and the males, only pollen cones. Whatever the species, a pollen cone always consists of an axis with stamens all around it. The stamens are small, stalked, scalelike organs with pollen sacs attached to them. In species belonging to the pine family, stamens are always numerous and are arranged in a spiral around the axis. In the cypress and yew families, the pollen cones have far fewer stamens, seldom more than a dozen (figure 3.2). The stamens are umbrella-shaped, with the pollen sacs on the inwardfacing , protected surfaces of the umbrellas. 36 the world of northern evergreens The pollen of conifers is familiar to anybody who has brushed against a cone-laden branch in spring, releasing dense clouds of yellow pollen. The quantity produced is enormous. It is borne on the wind to female cones (seed cones) awaiting fertilization. It blows everywhere and often collects as a yellow film on calm water surfaces—rain puddles, ponds, and the sheltered backwaters of lakes. Most people know pollen as “dust,” in airborne clouds or as waterborne films. The individual grains (see figure 3.2c for an example) are of much more than passing interest, however. A pollen specialist (a palynologist) Figure 3.1. Jack pine pollen cones. Figure 3.2. Pollen cones of (a) Douglas-fir and (b) Rocky Mountain juniper. (c) Pollen grain of a red pine. reproduction of conifers 37 can identify the genus, and sometimes the species, of tree that a pollen grain came from by examining its shape and texture.1 (Of course, if a pollen grain is collected for examination from a known tree, identification isn’t a problem.) Pines and spruces (prolific pollen producers) have comparatively large grains, each with a pair of bladders (floats) attached to it; figure 3.2c shows a grain of red pine pollen. These are the most easily identified conifer pollen grains. Those of larches, hemlocks, and Douglas-fir are bowl-shaped and much less distinctive. Unfortunately pollen grains are too small for their shapes to be visible with hand magnifiers. A large grain might be 50 µm across (that is 0.05 mm; 1 µm, a micrometer, is one-millionth of a meter, or 0.00004 in.). Palynology, the study of pollen deposits, is a science in its own right. It examines evidence on ancient climates and ancient vegetation. Pollen is produced in enormous quantities. That which settles on water eventually sinks to the bottom and becomes incorporated in the accumulating sediments. Sediment cores taken from these layers of mud contain pollen grains that were shed somewhere nearby, and many of the layers can be carbon-dated. The grains are almost indestructible. Long-buried ones are microfossils that persist unchanged, in recognizable form, for thousands of years. It is possible to infer from them how the vegetation of an area, and hence the local climate, have varied over past centuries and millennia. Pollination For conifers, pollination is the transfer of pollen grains from a pollen cone to a seed cone, where they will fertilize the ovules. The process is called wind-pollination, and the lightweight grains are easily borne on the gentlest of breezes. Some of the grains come to rest in unpollinated seed cones. All the cones described in chapter 2 had already been pollinated . Before pollination they were tiny, with soft, open scales between which pollen grains could sift down and reach the ovules. These “conelets ” are so small they are easy to miss. To find them, search carefully in early spring. In...

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Additional Information

ISBN
9780801463037
Related ISBN
9780801477409
MARC Record
OCLC
966802958
Launched on MUSE
2017-01-01
Language
English
Open Access
No
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