The architecture of the pistil and ovary (including its size and the number of ovules) differs among biological models. As described above, there are different types of stigma and style, and their sizes also differ considerably. For example, there are several types of placenta (placentation) such as parietal (e.g., Arabidopsis, Brassica spp. and corn poppy), axile (e.g., tobacco, tomato, petunia, lily, Torenia and snapdragon) and basal (e.g., rice and maize). In Arabidopsis, a typical placenta is not observed, and the placental tissue is usually called a septum. The number of ovules in one ovary is ~ 50 in Arabidopsis, ~ 3000 in tobacco, ~ 400 in lily, 1 in rice, and ~ 500 in Torenia.
The size of the ovule may affect the number of steps involved in pollen tube guidance by the embryo sac because the effective distance of chemoattraction is limited mathematically (Lush 1999). In the in vitro Torenia system and Ara-bidopsis, the effective distance of attraction by the synergid cell is only a few hundred micrometers at maximum (Higashiyama et al. 2003; Kasahara et al. 2005). In Arabidopsis, pollen tube guidance at the ovule is governed by the embryo sac, with at least two control steps (Shimizu and Okada 2000) but in larger ovules more steps may be necessary.
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