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A Close Up Look at a Black Oak (Quercus velutina) Leaf - Stellate Hairs
We have several large oaks in our yard- red oak, black oak, pin oak and white oak – that drop twigs now and then.
We decided to look at them more closely.
In the past we found some leaves that were in the black oak group but we had a difficult time identifying them. After some effort, we learned that black oak trees have sun and shade leaves that differ significantly. To the left is a sun leaf which has deeper sinuses than shade leaves. Also we have looked at the acorns of all our oak trees for positive identification of our black oak.
"Trichomes" is the technical term to describe small hairs that grow out from the leaf surface. There are many kinds: simple, stellate, glandular and more.
Leaves were collected 5/22/2019 and photographed.
The species name 'velutina' refers to the lower surface being covered with fine hairs which are especially thick on the veins and the axillaries. However, we came across an article that said the pubescence was stellate hairs and we decided to have a look. Here are some images from a regular camera and through a microscope.
This is a camera image of the stellate hairs on the upper surface.
This is a camera image of the stellate hairs on the lower surface. The lower surface is a lot more hairy especially on the veins and at the axillaries where it is more difficult to see that the hairs are stellate 
This is a dissecting microscope image of the stellate hairs on the upper surface.
The image below is through a compound microscope of a stellate hair that was scraped off a leaf.
This is an image through a compound microscope of the stellate hairs that were scraped off the leaves.
In general these trichomes can discourage herbivores, limit the effects of wind, and trap a layer of moisture to reduce water loss.
Trichomes help to avert herbivory by restricting insect movements or by storing toxic or bad-tasting compounds (black oak hairs are not glandular).
They can also reduce the rate of transpiration (water loss) by blocking air flow across and increasing the layer of still air around the leaf surface . They can help protecting the plant from wind and heat.
Leaf and stem trichomes increase the reflection of solar radiation, thereby reducing internal temperatures, and thus reduce water loss in plants growing under arid conditions. Trichomes can be insulating by keeping frost away from leaf cells.
Some references:
https://www.fs.fed.us/foresthealth/technology/pdfs/fieldguide.pdf
https://en.wikipedia.org/wiki/Quercus_velutina
H & M Ling
