▁of ▁x yle m ; ▁the ▁x yle m , ▁in ▁turn , ▁is ▁enc irc led ▁by ▁a ▁ring ▁of ▁ph lo em . ▁ ▁While ▁new ▁p ith ▁growth ▁is ▁usually ▁white ▁or ▁pale ▁in ▁colour , ▁as ▁the ▁t issue ▁ages ▁it ▁commonly ▁dark ens ▁to ▁a ▁deeper ▁brown ▁color . ▁In ▁trees ▁p ith ▁is ▁generally ▁present ▁in ▁young ▁growth , ▁but ▁in ▁the ▁tr unk ▁and ▁older ▁branches ▁the ▁p ith ▁often ▁gets ▁replaced ▁- ▁in ▁great ▁part ▁- ▁by ▁x yle m . ▁In ▁some ▁plants , ▁the ▁p ith ▁in ▁the ▁middle ▁of ▁the ▁stem ▁may ▁dry ▁out ▁and ▁dis integr ate , ▁resulting ▁in ▁a ▁h ollow ▁stem . ▁A ▁few ▁plants , ▁such ▁as ▁wal n uts , ▁have ▁distinct ive ▁chamber ed ▁p ith ▁with ▁numerous ▁short ▁cav ities ▁( See ▁image ▁at ▁middle ▁right ). ▁The ▁cells ▁in ▁the ▁peri pher al ▁parts ▁of ▁the ▁p ith ▁may , ▁in ▁some ▁plants , ▁develop ▁to ▁be ▁different ▁from ▁cells ▁in ▁the ▁rest ▁of ▁the ▁p ith . ▁This ▁layer ▁of ▁cells ▁is ▁then ▁called ▁the ▁per im ed ull ary ▁region ▁of ▁the ▁p ith am us . ▁An ▁example ▁of ▁this ▁can ▁be ▁observed ▁in ▁Hed era ▁hel ix , ▁a ▁species ▁of ▁iv y . ▁ ▁The ▁term ▁p ith ▁is ▁also ▁used ▁to ▁refer ▁to ▁the ▁pale , ▁sp ong y ▁inner ▁layer ▁of ▁the ▁r ind , ▁more ▁properly ▁called ▁mes oc arp ▁or ▁al bed o ,