Sexual attraction; here is the abstract:
Pollen tubes follow attractants secreted by the ovules. In a recent paper in BMC Plant Biology, Stewman and colleagues have quantified the parameters of this attraction and used them to calibrate a mathematical model that reproduces the process and enables predictions on the nature of the female attractant and the mechanisms of the male response.
Here is the last paragraph of the piece:
As with many other mathematical approaches to complex biological behavior, this new model from Stewman et al.  raises more questions than answers. But the fact that new approaches are contributing to a precise experimental description of the system [2,11,12] may make mathematical modeling an important tool for testing and selecting candidate molecules that may fit the in vivo biological profile of the final step of plant sexual attraction.
Here are the references in question:
 Dresselhaus T, Márton ML: Micropylar pollen tube guidance and burst: adapted from defense mechanisms? Curr Opin Plant Biol 2009, 12:773-780.
 Stewman SF, Jones-Rhoades M, Bhimalapuram P, Tchernookov M, Preuss D, Dinner AR: Mechanistic insights from a quantitative analysis of pollen tube guidance.
BMC Plant Biol 2010, 10:32.
 Márton M, Dresselhaus T: A comparison of early molecular fertilization mechanisms in animals and flowering plants. Sex Plant Reprod 2008, 21:37-52.
 Okuda S, Tsutsui H, Shiina K, Sprunck S, Takeuchi H, Yui R, Kasahara RD, Hamamura Y, Mizukami A, Susaki D, Kawano N, Sakakibara T, Namiki S, Itoh K, Otsuka K, Matsuzaki M, Nozaki H, Kuroiwa T, Nakano A, Kanaoka MM, Dresselhaus T, Sasaki N, Higashiyama T: Defensin-like polypeptide LUREs are pollen tube attractants secreted from synergid cells.
Nature 2009, 458:357-361.
Here is Reference 3. Have fun!