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67 Chapter 6 Two Kinds of Trees: Conifers and Broadleafs Introduction We have now taken a detailed look at the dominant trees in the northern evergreen forests—conifers—and a cursory look at the few deciduous trees that grow there with them—broadleafs. It is time to consider the contrast between the two kinds of tree. Not all conifers are evergreen (think of the larches), and not all broadleafs are deciduous (think holly). The differences between them are profound, so profound that it is ludicrous, nowadays, to lump them collectively as “trees.” Using one word for a combination of two such entirely different entities is inappropriate in the light of modern scientific knowledge. The Ancestry of “Trees” It may surprise many naturalists to know that conifers and broadleafs are less closely related to each other than are mammals and birds. The first conifers appeared about 300 million years ago, and the first flowering plants, about 140 million years ago1 (in what follows, the word “about” applies to every number). So flowering plants first appeared 160 million years after conifers. The first birds appeared 250 million years ago, and early mammals, that is, hair-growing animals that suckle their young like the primitive duck-billed platypus, 180 million years ago. So mammals came 70 million years after birds. 68 the world of northern evergreens If it’s legitimate to measure genetic divergence (or evolutionary change) by the time it takes to happen, the flowering plants differ from conifers more than mammals differ from birds. Botanists rest their case. Since angiosperms (including broadleafs) and gymnosperms (including conifers) became separated, both groups have continued to evolve and have left fossils, some of species still living and others of species now extinct . Recall that the angiosperms include all flowering plants, using the words in their botanical, not horticultural, sense: grasses and sedges, and broad-leaved trees with scarcely noticeable flowers belong to the angiosperms . The gymnosperms include, besides the conifers, two other groups, both semitropical, that reproduce in the same way. First is the exotic maidenhair tree (Ginkgo biloba), which is often grown as an ornamental and is the only surviving member of a larger group whose other members are now all extinct. Second is the cycads with several species; a typical cycad looks like a dumpy palm tree with big cones, and is often grown as a hothouse ornamental. It seems likely that conifers and their relatives are on their way to extinction . Their numbers and variety appear to have been dwindling for millions of years. The world is poorer in conifer species than it was millions of years ago, and as their variety gradually diminishes, flowering plants seem to be diversifying. This is the reason for believing that flowering plants have evolved more than conifers. They have been able to adapt faster to ever-changing conditions. Or, what comes to the same thing, nature is selecting them. The Basic Difference between Conifers and Broadleafs The greatest contrast between conifers and broadleafs is in their reproductive systems. One anatomical difference was described in chapter 3: the ovule in a cone is naked in the sense that the pollen tube has direct access to it. The ovule in a flower is inside a carpel, a soft outer covering that the pollen tube has to penetrate to reach the integument and ovule. But that is only half the story, the less interesting half. The other half is that whereas gymnosperms need only one sperm cell to fertilize an ovule, angiosperms need two. Figure 3.4 shows this most surprising difference , as well as that between the layers of tissue the pollen tube must penetrate, as already described. two kinds of trees 69 Skip the next three paragraphs if you don’t want the technical details. First, a reminder on the chromosomes of sex cells: In flowering plants, as in humans and other mammals, every cell in the body except for the sex cells contains a duplicate set of chromosomes; such a cell is called a diploid cell. The sex cells, on the other hand, both male and female, each contain only a single set of chromosomes and is called a haploid cell. Therefore, when a sperm fertilizes (unites with) an egg, the result is a diploid cell, the zygote, which is the first cell of an embryo. The zygote divides and redivides repeatedly, eventually becoming a ripe seed, a baby, a puppy, or whatever the case may be. Now refer to figure 3.4. Figure...


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