This shows that even as seeds the plants recognise what kind of folk live next door.
The scientists behind the study believe sounds generated by microscopic movements within plant cells might be the key to the mystery, the Daily Express reported.
Fennel is a bad plant neighbour because it competes aggressively, releasing chemicals that stunt the growth of its rivals.
Basil, on the other hand, produces helpful chemicals that keep out weeds and act as natural insect pest killers.
Dr Monica Gagliano, from the University of Western Australia, who is the lead researcher of the new study, had earlier found that, even when protected against fennel's chemical attack, chilli seeds are reluctant to germinate when placed near the plant.
For the latest study, chilli seeds were planted in isolated dishes around an adult basil plant that was either left open to the elements, or sealed in a cylindrical box covered in black plastic.
The cylinder blocked all airborne chemical signals, and all wavelengths of light. As a comparison, other seeds were planted around a cylinder that was left empty.
More of the chilli seeds sprouted and grew when the basil neighbour was present, whether or not the plant was masked and cut off from them.
The scientists stated that the presence of basil positively enhanced germination rates of chilli seeds, validating the claims of many gardeners who recognise the beneficial effect of basil on the growth of chilli plants.
The fact that germination of chilli seeds did not differ between the masked and open treatment indicates that light, touch or chemical signals may be important but clearly not necessary for chilli seeds and basil plants to sense each other's presence, they noted.
The study results were published in the online journal BMC Ecology.