91热爆

Deserts can remain dry for long periods but when rain does come it can even activate dead stems so that within half a minute they are standing erect once more. Other plants open their seed heads when it rains. Yet none of the plants are alive. Their movement is simply the effect of dead tissues absorbing water. The dead seed heads hold the seeds securely during the drought. But now that there has been rain and therefore a chance of them being germinated, it is time for them to be distributed. For some plants the heavy rain drops are enough to dislodge the seeds, but other plants use the physical effects of absorbing water to shoot the seeds away. Once those seeds land, they too start to absorb water so that the seeds end up in just the right position to send their roots straight down into the ground.