are very similar genetically. "And that's a real problem when you're trying to overcome new insect or disease threats or pressures, or trying to breed a plant that's more water efficient, or even finding genetic variation for water production."

VOA: standard.2010.02.01

So, instead of having the periodic variation of water, or the periodic variation of air density here we're talking about an electric field.

不像水或者空气密度的,周期变化，我们这里,讨论的是电场。

麻省理工公开课 - 化学原理课程节选