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Section 4 Diversity
The ovulate of pines are much larger and more complex in their structure than are the pollen-bearing cones (Figure 20-23).The ovuliferous scales(cone scales),which bear the ovules, are not simply megasporophylls. Instead, they are entire modified determinate branch systems properly known as seed-scale complexes. Each seed-scale complex consists of the ovuliferous scale—which bears two ovules on its upper surface—and a subtending sterile bract (Figure 20-24).The scales are arranged spirally around the axis of the cone. (The ovulate cone is, therefore, a compound structure, whereas the microsporangiate cone is a simple one, in which the microsporangia are directly attached to the microsporophylls.)Each ovule consists of a multicellular nucellus (the megasporangium) surrounded by a massive integument with an opening, the micropyle, facing the cone axis (Figure 20-24).Each megaspore mother cell, which ultimately undergoes meiosis, giving rise to a linear series of four megaspores. However, only one of these megaspores is functional; the three nearest the micropyle soon degenerate.
Pollination in pines occurs in the spring (Figure 20-25).At this stage, the scales of the ovulate cone are widely separated. As the pollen grains settle on the scales, many adhere to pollination drops, which exude from the micropylar canals at the open ends of the ovules. As the pollination drops evaporate, they contract, carrying the pollen grains through the micropylar canal and into contract with the nucellus. At its micropylar end, the nucellus is slightly suppressed. The pollen grains come to lie in this shallow cavity. After pollination, the scales grow together and help protect the developing ovules. Shortly after the pollen grain comes in contact with the nucellus, it germinates to form a pollen tube. At this time, meiosis has not yet occurred in the megasporangium.
About a month after pollination, the four megaspores are produced, only one of which develops into a megagametophyte. The development of the megagametophyte is sluggish. It often does not begin until some six months after pollination, and even then may require another six months for completion. In the early stages of megagametophyte development, mitosis proceeds without immediate cell wall formation. About 13 months after pollination, when the female gametophyte contains some 2000 free nuclei, cell wall formation begins. Then, approximately 15 months after pollination, archegonia, usually two or three in number, differentiate at the micropylar end of the megagametophyte, and the stage is set for fertilization.
那松树排卵更非常大而且更多复杂的在他们的结构比较是举止花粉的圆锥体 (图 20-23).ovuliferous 刻度 (圆锥体刻度),忍受 ovules,不只是 megasporophylls 。 相反地，他们是整个的修正适当地即是种子-刻度的合成物的确定的部门系统。 每个种子-刻度的合成物有 ovuliferous 刻度-在它的上面表面上忍受二 ovules-和一次倾向不毛的苞叶 (图 20-24).刻度在圆锥体的轴的周围螺旋形地被安排。 (那排卵圆锥体因此，是复式的结构，然而 microsporangiate 圆锥体是简单的，在那里 microsporangia 直接地被附上到 microsporophylls.)每 ovule 有被一个巨大外壳用一个开始， micropyle 包围的多细胞 nucellus(大孢子囊), 面对圆锥体轴 (图 20-24) 。每 megaspore 母亲细胞,最后接受减数分裂, 引起线连续的四 megaspores。 然而，这些 megaspores 的只有一个是功能的; 那三最接近 micropyle 很快退化。
在松树的授粉在春天 (图 20-25) 发生。在现阶段, 刻度那排卵圆锥体广泛地被分开。 作为在刻度上的花粉谷粒长椅，多数黏附在授粉下降,在 ovules 的开着结束从 micropylar 运河流出。 当做授粉下降蒸发，他们缩短, 经过 micropylar 运河携带花粉谷粒和进入契约之内以 nucellus。 在它的 micropylar 结束， nucellus 些微地被镇压。 花粉谷粒开始在这水浅的地方洞中。 在授粉之后，刻度一起扩大，而且帮忙保护发展中的 ovules 。 在不久之后，花粉谷粒用 nucellus 进来连络，它发芽形成花粉管。 在这次，减数分裂在大孢子囊中还没有发生。
在授粉后大约一个月，这四 megaspores 被生产, 哪一个进入 megagametophyte 之内发展的只有一个。 megagametophyte 的发展是偷懒的。 它时常直到才开始, 而且甚至然后可能为完成需要另外的六个月。 在 megagametophyte 发展的早阶段中, 有丝分裂没有立即的细胞壁形成的收入。 在授粉后大约 13个月，当女性的配偶体包含一些 2000个自由的核的时候，细胞壁形成开始。 然后, 在授粉后 15个月大约, archegonia, 通常二或三在数字中，在 megagametophyte 的 micropylar 结束区别, 而且阶段为施肥被设定。
