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 <WSD>maturity</WSD> at , let 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:  the period of time in your life after your physical growth has stopped and you are fully developed