Basic Botany: Xylem and Phloem


I first learned about xylem and phloem back in school … a half-century ago! So, I’m due for a bit of revision … and maybe you are as well. 

Xylem and phloem are conduits, part of the vascular system of higher plants: they’re designed to move water, minerals and sugars (food) from one part of the plant to another. 

“Lower plants,” like mosses and liverworts, simply count on capillary action as their source of water movement, like when you dip the tip of a cloth in water and then moisture moves further up the cloth, but then they’re only a few centimeters high at most. That won’t work for taller plants. Imagine getting water to flow up to the top of a giant redwood simply by capillary action! Instead, specialized cells line up together and transfer the products up and down, as needed. 

Of course, try sending one product up a tube while another comes down the same tube at the same time in the opposite direction with any speed. That wouldn’t work! So higher plants developed two types of cells: xylem and phloem.

Xylem cells do the hard work, “pulling” water and minerals up, from one cell to the next, from the roots all the way up the stem to the very top of the plant. Rich in lignin, they also form the wood of trees and shrubs. The rings you see in a tree stump are made of xylem.

Phloem cells don’t work quite the same way. Sugars, resulting from photosynthesis and thus mostly produced by the leaves, are needed by all parts of the plant, from the flowers at the top to the roots at the bottom, so they have to flow both up and down and do so by diffusion … and also by gravity (most of the sugars eventually reach the roots). Phloem cells take the sugar-rich sap to where it is needed through their perforated cell walls.

These cells are found in all parts of the plant.

I recall in school understanding the concept, but not being able to remember which was which, which was always stressful when a biology exam was coming up. I wish someone had taught me the following mnemonic: water zips up the xylem and flows down the phloem. So simple!

Larry Hodgson is one of Canada’s best-known garden communicators. After studies at the University of Toronto and Laval University where he obtained his B.A. in modern languages in 1978, he succeeded in combining his language skills with his passion for gardening in a novel career as a garden writer and lecturer. He has notably been editor-in-chief of HousePlant Magazine, Fleurs, Plantes et Jardins, À Fleur de Pot and Houseplant Forum magazines and is currently the garden correspondent for Le Soleil and radio garden commentator for CKIA-FM Radio. He is a regular contributor to and horticultural consultant for Fleurs, Plantes, Jardins garden magazine and has written for many other garden publications in both the United States and Canada, including Canadian Gardening, Harrowsmith, Horticulture, Fine Gardening, Rebecca’s Garden and Organic Gardening. He also speaks frequently to horticultural groups throughout Canada and the U.S. His book credits include The Garden Lover’s Guide to Canada, Complete Guide to Houseplants, Making the Most of Shade, Perennials for Every Purpose, Annuals for Every Purpose, and Houseplants for Dummies, as well as nearly 50 other titles in English and French. He can be seen in Quebec on French-language television and was notably a regular collaborator for 7 years on the TV shows Fleurs et Jardins and Salut Bonjour Weekend. He is the President of the Garden Writers Association Foundation and the winner of the prestigious 2006 Garden Media Promoter Award offered by the Perennial Plant Association. An avid proponent of garden tourism, he has lead garden tours throughout Canada and to the gardens of over 30 countries over the last 30 years. He presently resides in Quebec City, Quebec.

1 comment on “Basic Botany: Xylem and Phloem

  1. Ha! So I am not the only one who confuses them. I remember that wood is ‘secondary xylem’, which reminds me sort of where active xylem is located. Also I remember that two way traffic is on the outside of that.
    Then we have palms.

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