Replies

1. 
   jonsherryl | Apr 25, 2003 09:16pm | #1

   Brill, reaction wood is abnormal wood tissue produced by the tree in an effort to alter the direction of growth of the trunk or branches. Through methods not yet well understood, the terminal buds of the tree produce growth hormones that are somehow directed in differing amounts back along the the vascular tissue between the cambium layer and the inner bark to the point along one side of the trunk or stem where growth must be either accelerated or retarded in order to cause a change in the direction of growth. It's kind of like how a tank turns by changing the speed of the tread on one side or the other. The primary purpose of this process is to allow the tree to better position its foliage to capture sunlight when obstructions occur in its immediate environment.

   The more primitive gymnosperms (softwoods) and the more advanced angiosperms (hardwoods) have "elected" opposite strategies for accomplishing this task. The softwoods achieve it by creating compression wood on the inside of the bend, while hardwoods create tension wood on the opposite (outside) of the bend. reaction wood of either type has radically different (much higher) longitudinal shrinkage than does normal wood tissue...and this often leads to warping and cross-grain checking as the wood is seasoned. Also, the porosity of reaction wood differs from that of normal wood tissue, causing blotches when the wood is stained.

   As for there being a difference between how this process would function based on which hemisphere the tree is growing in is a new one on me. It doesn't seem to make sense that it would and, if there is any reseach to that effect, I've missed it.

   
   

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   Edited 4/25/2003 2:21:06 PM ET by Jon Arno

2. 
   kayl | Apr 25, 2003 09:25pm | #2

   Botanist here, not a wood anatomist... but I cannot explain the

   statement quoted except by saying that I think something got turned

   around someplace, and not with you!

   Gymnosperms (softwoods) make compression wood on the underside

   of branch/trunk junctions; Angiosperms (hardwoods) make tension wood on the upper side of branch/trunk junctions. All the herbarium specimens of woods I've seen from the southern hemisphere had the

   same orientation of reaction wood as species from the northern

   hemisphere, so I think you're on solid ground here.

   Auxins and ethylene (two classes of plant hormones) appear to be

   part of what controls the formation of reaction wood. When gymnosperms have high concentrations of auxins localized in the

   low side of a tilted bit of wood, you get compression wood (on the

   lower side). Low concentrations of auxins elicit tension wood

   formation (upper side) in angiosperms.

1. 
   jonsherryl | Apr 26, 2003 12:06am | #3

   Kay, you're right.  I know better, but I have "inside" and "outside" reversed in my post. Thanks for the correction.

   Edited 4/25/2003 5:25:34 PM ET by Jon Arno

1. 
   pgwill | Apr 26, 2003 02:02am | #4

   Jon and Kay, thanks very much for the prompt and informative responses.

   That's waht's really great about this forum.