Dear David,
Thank you very much for your response! - The Salix nigra Marsh. leaf that I 
attached to my previous message is from a herbarium voucher from Muzquiz, 
State of Coahuila, Mexico. The label says that it was collected in the “open 
valley floor”. That is all the information I have. George Argus (in Flora of North 
America) gives the following habitat types for that species: floodplains, edges 
of ponds and lakes, swamps, marshes, white cedar bogs, wet meadows, open 
fields, roadside ditches, and mixed upland deciduous woods along streams. I 
therefore assume that water availability is not an issue in places where S. 
nigra is naturally growing. The balance between water availability and 
evapotranspiration could, of course, be disturbed in an extreme dry-air 
climate. The climate in Muzquiz is very dry (ave. yearly precipitation 6.76 
inches), but another S. nigra leaf from the East Coast of Florida (Jacksonville, 
ave. yearly precipitation 52.75 inches) has a venation pattern similar to the 
one from Mexico.
If salicoid teeth are thought to function as “overflow valves”, then I don’t 
think that this venation pattern (intramarginal vein combined with a densely 
toothed margin) would conserve much water. The teeth are still connected to 
the secondary vein system (via tertiary gauged veins). In this respect the 
venation pattern is not that different from a eucamptodromous pattern 
(except for the added intramarginal vein). To conserve water the number of 
teeth would have to be significantly reduced, which is not the case in the 
Muzquiz specimen or any of the others.
I have reason to assume that the occurrence of intramarginal veins (in S. 
nigra) at low latitudes is not the result of an evolutionary step-by-step 
adaptation to a different climate. A S. babylonica L. plant from Langtang 
Valley, Nepal (alt. 3500 m) produced  a eucamptodromous venation pattern  in 
its original growing place and an intramarginal vein after being transplanted by 
cuttings to Boise, Idaho (alt. 850 m). After a second move to a slightly higher 
elevation (ca. 950 m) it returned to a eucamptodromous venation pattern. 
The first Boise location had a rich topsoil and flood irrigation. The second 
location has an artificially enriched sandy substrate and drip irrigation. Growth 
rate in the first (Boise) location was somewhat higher than in the second one, 
but both plants are thriving well. This finding suggests that venation pattern is 
more flexible than previously thought and may be responding to some 
environmental factors.
I would like to check the latitude dependence of the venation pattern in 
another subg. Protitea species, S. humboldtiana Willd. Its natural distribution 
ranges from northern Mexico to southern Chile (Patagonia). Salix humboldtiana 
does develop intramarginal veins, but so far I have only seen leaves from 
Mexico and from a place near Buenos Aires, Argentina. If somebody has 
access to herbarium material from northern and southern Chile I would very 
much appreciate one or two mature leaves from each plant.
Concluding, I would like to repeat my original question: Has anybody seen 
fossil (Cenozoic) Salix or Salix-like specimens with intramarginal veins? 
Sincerely,
Walter Buechler