In pea and at least some monocotyledonous species, both the generative cell and the sperm cells regularly contain plastids. Nonetheless, these species do not normally transmit paternal plastids into the zygote. It is generally believed that, dur ing fertilization of the egg cell by one of the two sperm cells, the plastids are stripped off together with most of the cytoplasm and do not enter the zygote along with the sperm cell's nucleus (Hagemann and Schröder 1989; Fig. 2; Table 2). Such a mechanism would be somewhat reminiscent of the exclusion of sperm cell mitochondria during fertilization of the egg in animals. However, 'smoking gun' evidence for a stripping-off mechanism underlying maternal inheritance in the Triticum type is largely lacking. This is mainly due to the difficulty to catch in the act sperm and egg by electron microscopy. Therefore, alternative mechanisms, such as degeneration of the cytoplasm surrounding the sperm cell nucleus (including the demise of plastids and mitochondria) shortly before the fertilization process, presently cannot be excluded.
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