Why does some seeds germination need the ray?

[Miyis]

Burdock and some weed ,they seed germination need the ray.

please tell me: Why does some seeds germination need the ray?

[Belana]

Every succesfull seed must once find out when there is the right time to germinate. There are many strategies how to recognize this time. For many plants, the right humidity and temperature for definite time, is enough - so they start to germinate. But in some areas, seeds need special impulses. It is due to extreme conditions (many factors are influential in growth). What about some australian plants that need a fire for germination? I am sure that the need of rays stems from fact, that these rays symbolize a newly conditions set in.