Ovule attraction and the chemotaxis of pollen tubegrowth. (a) Pollen tube guidance precedes double fertilization in flowering plants. A pollen tube carrying two sperm cells leaves the placenta to grow along the funiculus (the foot of the ovule) into the micropyle (the entrance of the ovule) following gradients generated by the maternal tissues of the ovule and by the female gametophyte. An embryo sac contains the egg apparatus (egg cell and two synergid cells), the central cell with two polar nuclei, and three antipodal cells. It is usually surrounded by a supportive tissue - the nucellus - and two layers of protective tissue - the inner and outer integuments. In Torenia the nucellus is disintegrated, generating a naked egg apparatus at the micropylar region. Adapted from . (b, c) Semi-vivo growth system in Arabidopsis. Pollen is germinated in the stigma, but the style is cut (top in (b)) and co-cultivated with dissected ovules (bottom in (b)). When coming out of the style, pollen tubes grow in the surface of a semi-solid agar medium, and eventually target the micropyles of the ovules (c). If penetration is achieved, the contents of the tubes are discharged inside one synergid; if the system is carried out with pollen tubes (arrows in (c)) labeled with green fluorescent protein, the moment of fertilization is visible by fluorescence (arrowheads in (c)), and ovules can be scored in terms of successful attraction. The scale bars represent 100 mm. Adapted from . (d) Depiction of the angles used in the analysis of pollen tube turning made by Stewman et al. . These angles indicate how much the pollen tube would have to turn to take the most direct path toward the micropyle (qmp), and describe the new direction chosen by the pollen tube in response to the gradient (qtip). These quantitative data were then gathered for various incubation periods to deduce the nature and effect of the gradient produced by the diffusion of an attractant from the ovule's micropyle.