http://en.wikipedia.org/wiki/Phloem
http://www.uic.edu/classes/bios/bios100/lecturesf04am/lect19.htm
http://virtualplant.ru.ac.za/Main/FACTFILE/phloemtrans.htm
https://www.msu.edu/~walwort8/page2.html
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/Phloem.html
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