Subject: What's the cause of Crookes Dark Space & striations in discharge tubes

I am a science teacher who has looked high and low for the ansewr to this 
question in books and on the 'net. All high school textbooks say (if they 
even mention cause) is that the reasons for these phenomena 
are "theoretically complicated". Not very helpful. I have had no more luck 
with university texts or high-end encyclopedias. Sometimes the phenomenon 
is mentioned, even noted as important, but never explained. To follow my 
syllabus I must demonstrate these features -- and that's it! I'm sure some 
of my more curious students will ask "why". Please help me to give a 
better answer than "I don't know" or "You don't need to know". Thanks.

Re: What's the cause of Crookes Dark Space & striations in discharge tubes