I wrote a list of different functions and script and I put them in some subfolders of the working directory so I can divide all my functions in arguments (descriptive statistic, geostatistic, regression....)

When I type source("function\_in\_subfolder") R tells me that there is no function. 
I understood that it happens because functions have to stay in the working directory.
Is there a way to set also subfolders of the working directory as source for the functions (let's say in a hierarchical way)?