A New Category

One of my readers recently had a good idea for a new category on the blog. His idea was to explore the fascinating world of Tropisms. What are they? Tropisms are the ability to respond, usually by growing toward or away from a particular stimulus. The Latin root “tropism” means “turning.” This refers to the plant turning towards, known as positive, or turning away, known as negative. Plants and fungi have many tropisms and are fascinating.

One of the most common tropisms in plants is positive phototropism, the ability of plants to grow towards light. Everyone knows about this phenomenon but we seldom think about how and why they do it. Usually plants respond to a stimulus because it will increase their chances for reproduction. At the simplest level, a plant grows toward the light so it will get more sunshine and grow bigger and stronger and bear a more prolific cone or fruit or berry with lots more chances to make little baby plants. But how is it done, exactly?

Well, we know some ways it is done and other ways we can only guess. The first and most obvious lesson in tropisms is the classic experiment where a seed is germinated on its side and we can observe the radical (shoot) grow up and the root, grow down. This experiment is done in the dark. This is geotropism, also known as gravitropism. The words mean earth-turning and gravity-turning, respectively. It is usually called geotropism in most classes. The shoot responds by growing in the negative direction from the earth and the root grows toward the earth. The shoot is negatively geotropic and the root is positively geotropic.
This is very handy for a seed covered with soil and starts the little plant on its way to having the roots down and the shoots up.

How does the plant detect the gravity? No one knows. We can guess that the plant hormone auxin is somehow associated with starch molecules that settle on the downward side of the plant. Strictly speaking the starch molecule theory has been disproven but some mechanism tells the auxin hormone to go to the bottom side of the plant. We do know that auxins have opposite effects on the plant parts. The shoots respond by growing away from the auxin heading the new shoot upward away from gravity.

The auxin hormone, on the other hand, makes roots grow towards itself. Down the root tip goes, down into the soil where it belongs, if you are a root. This is the way new plants get headed in the correct directions. This experiment is a classic botany lesson first described by Charles Darwin.

There are many other tropisms in plants and fungi involving different stimuli. I will try to do a occassional post on some of the more interesting ones.

Thank you to my reader who has started an interesting area of blogging.


Ivy Growing up a Maple Tree

I must comment on an article I recently read in a free gardening newsletter found in a diner. I hope this is not where most gardeners find their information. In the newsletter there was a question and answer column with highly suspect advice. The questioner wrote in asking if English Ivy

    Hedera helix

was hurting his maple tree as it was climbing up the trunk. I think the question was, maybe, wondering if the vine was some kind of parasite.

I have observed ivy climbing many houses, trees, fences, etc. and I can fairly well state that unless the tree is really weak and would fall down from the weight of the vine, it is really fine. The vine is just using the tree, or whatever, as support and is in no way a parasite*. It also cannot possibly overwhelm a big tree like a maple or oak.

Let me quote the fine answer this columnist gave:

“The plant is an aggressive climbing vine.”…”As the ivy climbs in search of increased light, it consumes and kills branches by blocking light from the tree’s leaves.”

I have never seen this occur. I have, however, seen ivy with no damage to the branches/trees for over fifty years of growing up the trunks. The columnist does not mention one slight problem with vines growing up a tree trunk and that is gypsy moth.

In the “old days” when gypsy moth was in its glory days and just moving into the midwest, it munched on many species of trees in residential areas. After feeding, the females seek a secret area to lay eggs. Since the pregnant females can hardly walk, they are so full of eggs, they like to walk down the trunks of trees and lay eggs under the cover of vines which are growing up the trunk. I would often find eggmasses hidden amoungst English ivy leaves.

This is

    not now

a real problem since gypsy moth has settled into a more moderate pest status with predators, parasites and diseases keeping it in check.

Back to this fine article:

“On the ground, English Ivy forms dense monocultures that exclude native plants, not just weeds. It also serves as a host for bacterial leaf scorch, a pathogen harmful to elms, oaks, maples, and other native plants.”

It does neither. I suppose this author has never supervised a landscape crew pulling weeds and “native” plants out of a bed of ivy. As for the unnamed bacterial leaf scorch, I know of no pathogen that attacks that wide of an array of plants and certainly not that array and ivy doesn’t get anything. Bizarre.

This goes on:

“Maple roots…With a mat of ivy on top are short-changed of water, and nutrients.”

Well, except for the fact that a maple tree has roots going all over in a very wide pattern and the ivy is usually in a small planting bed around the tree. I guess if the ivy was planted in a really huge bed, it would steal food from the tree about 5% but it doesn’t have the ability to block water that much. As for nutrients, that tree will get food just fine. The ivy is not that heavy a feeder.

I am weary of this odd question and answer. One of the things I have always disliked about gardening is there is a lot of wacky advice out there. I will put this article in my category of “wives’ tales.” Here is my advice to my loyal readers, do not get your gardening information from your local diner.

*Regarding parasitic vines, the reader may want to view a previous post on the true parasitic plant, Dodder.