I N T R O D U C T I O N
The life cycle of a pine (cont'd)
7 By the time eggs are ready to be fertilized, two sperm cells have developed in the male gametophyte (pollen grain) and the pollen tube has grown through the nucellus to the female gametophyte. Fertilization occurs when one of the sperm nuclei, injected into an egg cellby the pollen tube, unites with the egg nucleus. All the eggs in an ovule may be fertilized, but usuallyonly one zygote develops into an embryo. 8 The pine embryo, or the new sporophyte, has a rudimentary root and several embryonic leaves. A food supply, consisting of the female gametophyte, surrounds and nourishes the embryo. The ovule has developed into a pine seed, which consists of an embryo (new sporophyte), its food supply (derived from the gametophyte tissue), and a surrounding seed coat derived from the integuments of the parent tree (parent sporophyte). Notice that three plant generations - one gametophyte and two sporophyte generations - are represented in a gymnosperm seed (Campbell et al, 1999).
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