I know that "shrub" is a countable noun. However, I still find that people use "shrub" with "s" and no "s". I have learned this from many articles on some webs. Just type "desert shrub" and you will find them. I want to ask one question. "Desert shrub have tiny leaves that lose little moisture on a hot day". Is this sentence correct? "shrub" here is used without "s". Many thanks.