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 , let us guess , 18 inches longer than the others , a <WSD>quadrupling</WSD> of the maturity length would result in a maximum of ( nearly ) 40 feet . 
Meaning of the word marked with <WSD> tags:  increase by a factor of four