Roots

[jmmehler]

A friend of mine and I were discussing the condition of phalaenopsis roots, and started to devise some theories. Since I was not able to find any information I’d like to post the question here. By examining the rings in a cross section of the tree trunk one can determine the years in which the tree did well, or poorly. In a similar fashion, by examining the condition of orchid roots, we should be able to make some similar determinations as well.

1) When sections of the root are constricted, this must indicate some sort of stress to the root and or the plant. Can anyone comment as to the reasons for these constrictions?

2) When that constriction is so severe, that the root breaks, but continues to hang on by a thread, I have noticed that often times the section beyond the break is still very green. Does that thread still transport water and nutrients to the plant even though the root is broken?

Any help would be appreciated.

[tucker85]

The tiny thread in the center is the actual root. The root is covered with a spongy material called velamen. Even if the velamen is destroyed or degraded, the root itself can still be alive and can function to carry water to the orchid. Of course it is possible to kill the root itself. In which case it won't function. I'm not sure what causes all the constricted areas on a root. My suspicion is that the orchid survived a period where there wasn't enough moisture or humidity and the root just dried up. But some insects can damage roots and they can also be mechanically damaged .

[Ray]

Actually, the root is more complex than that. That central "thread" is the stele, which contains the vascular structures (xylum and phloem) that transport water and nutrients.

Here's a similar discussion: Firm Brown Roots

Changes in the outer parts of the roots can be due to any number of environmental factors.

[jmmehler]

Thank you Tucker and Ray. Your responses have been very helpful. Now that I have a name, velanum, I've read up some on the subject.

The topic still begs some questions. during repotting, is it better to cut away roots with broken velanum to avoid diseases, or would the actual cut expose you to more disease, as the broken velanum may already have 'scabbed' over.

Also can we read anything into the widening and constricting of the velanum, along the length of the root?

[naoki]

Tree rings are coming from "secondary growth", which means the growth of girth/width. The growth is coming from the cell division of cells between phloem and xylem. But orchids roots are from the cell division of the root tip (root epical meristem). So it is fairly different process. Most monocots do not have secondary growth (are there any monocots with secondary growth?).

After re-reading the question, your "constriction" probably means the skinny section of roots with uneven thickness. Then it is somewhat related to speed of the growth and also is is influenced by the obstacles (e.g. small holes) the roots has to go through. If the root growth slows down/stops due to some events (not enough water, light, temperature, or they have to put resource to other tissues like leaves or flower), the roots grown around that time seem to become somewhat skinnier in some species. I'm not sure if you can say that it is caused by "stress".

I thought that the number of velamen layers is more or less determined by the genetics (different species have different number of layers).

You wouldn't cut off your leg when you scratch your knee, would you? ;-)