Conceptual “cartoons” of the effects of reproductive-organ height on the transfer of pollen between distylous flowers.

<p>Arrows show directions of pollen flow, arrow widths show magnitude of expected fertilization, and dashed red circles indicate the region with the highest probability of pollen deposition. A: Pollen transfer between well-matched reciprocal morphs. Pollen donated at the low position by a pin is transferred to a low position on the pollinator's body and arrives at a low position in the recipient thrum; similarly, pollen donated at a high position by a thrum arrives at a high position in the recipient pin. Because pollen arrives at the height of the recipient stigma and is compatible with it, fertilization is likely to occur. B: Hindrance of pollen transfer between reciprocal morphs poorly matched in their reproductive-organ heights. Due to this mismatch, pollen arrives at the wrong height and is thus less likely to be received by the stigma and result in fertilization. The height mismatch thus causes some degree of reproductive isolation.