Sentence:  If , in a certain part of the range , it starts life 1 foot longer than do any of the other ( relatively large ) giants , and reaches maturity at , <WSD>let</WSD> us guess , 18 inches longer than the others , a quadrupling of the maturity length would result in a maximum of ( nearly ) 40 feet . 
Meaning of the word marked with <WSD> tags:  make it possible through a specific action or lack of action for something to happen