Sentence:  Jubal said that his night-sight probably came from the conditions in which he had grown up , and Mike grokked this was true but grokked that there was more to it ; his <WSD>foster</WSD> parents had taught him to see . 
Meaning of the word marked with <WSD> tags:  a person who acts as parent and guardian for a child in place of the child's natural parents but without legally adopting the child