Can I get bower_components by `require_directory` ?

[kntmrkm]

I can get css by below

*= require cssgram/source/css/1977

but I don't know for require_directory

*= require_directory cssgram/source/css
or 
*= require_directory ./cssgram/source/css/

That say require_directory argument must be a relative path.

Thanks.