Question
my aspen tree has leaves that have pea shaped things that are on the back of the leaf along the main stem. They are in a row like a row of peas. They are light green in color. The leaf is off the tree and seems dry like the moisture is being sucked out of it. Several of the leaves on the tree have this and this has occurred for a few years.

Answer
Judy:
These structures may be an insect gall called Phylloxera.  Google this word for pictures to look at.  Usually this insect has minimal impact on the overall health status of the tree, assuming it is watered and fertilized well.  Also helpful is to rake up and destroy leaves that fall since the insect can spend the summer or winter on these fallen leaves, so it may help reduce the incidence for next growing season.

Regards
Steve