Phloem -- Philosophically Speaking

It was a simple challenge from a friend -- "What is phloem?"

Within each tiny plant is a microcosm of our world

I could have ignored his question. But that didn't seem right, given that my shelves are crowded with books on gardening -- garden design, garden pests, garden techniques, gardeners talking to other gardeners.

Aside from the space these tomes require, buying books is no small investment. But when you love gardening, it's tough to bypass a book with a delicious cover. So, as an exercise in self-discipline, I've been acquiring used gardening books for little or nothing.

Sometimes it's the writing that draws me in, sometimes the artwork. Occasionally just the look and feel of the book itself attracts me. I wonder how many other gardeners have leafed through its pages.

Botany for Gardeners

That's how a book called Botany for Gardeners by Harold William Rickett came to sit on our shelves. Published by MacMillan in 1957, its apple green cover protects a handy little reference gem that lay dormant, until?

Phloem?What is phloem?

Mostly what I remember from high school biology had to do with those poor little frogs. So a botanical refresher course was in order when my friend posed his question.

I soon discovered that Mr. Rickett writes not about how to grow plants, but about how plants grow.

To understand phloem, you need to remember a couple of things:

  • First, chloroplasts carry chlorophyll. The illumination of these living green cells transforms sugars into starch.
  • Second, carbon dioxide taken up from air and water is a critical component of the process,

But once created, how does this "food"-- starch -- travel down to the roots and out to the flowers and fruits?

Plant transportation

Oversimplified, herbaceous plants have two transportation systems:

  • The xylem brings water and inorganic ions up from the roots and out to the leaves
  • The phloem sends the processed food out to the plant.

The phloem is not hollow like a straw, but instead consists of long fibrous tissues stacked on top of one another called sieve tubes. These and some companion cells that lie alongside the sieve tubes comprise the phloem. It is this phloem tissue that transports the sugars, amino acids and hormones.

Despite Mr. Rickett's thorough explanation, I needed a little more visual edification and logged on to the Botanical Society of America website, www.images.botony.org/bsa. This site includes hundreds of botanical pictures on various topics. I also visited my local library and checked-out a book called Plantwatching, by Malcolm Wilkins, published by Roxby Reference Books in 1988. Mr. Wilkins' pictures and diagrams were terrific.

Those of you who are fascinated by old house heating systems will love his explanation of the phloem's operation: A negative pressure system in which suction is the driving force, as compared to the xylem, which acts as a positive pressure system.

A microcosm of our world

I'll confess, the phloem question was a challenge. But the more I researched the question, the more I liked thinking about phloems.

It was a gentle reminder that while I'm busy focusing on the big picture of major perennial garden renovations and fencing to keep marauders out, a microcosm of our world is inside each little plant.

This invisible world operates quietly, efficiently. And were it not for that occurring billions of times each day all around the world, life would have a whole other definition, literally.

Thank you Mr. Rickett for your lovely book. As I always say ? you can never have too many gardening books.

J. B. McGowan is an avid gardener whose midcoast Maine perennial gardens have been featured in local publications and on house and garden tours.

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