The seeds of oaks trees are acorns. The acorn is defined in the book, "Plant Identification Terminology" by James G. Harris and Melinda Woolf Harris as," The hard, dry, indehiscent (which means that they do not open at maturity along definite lines) fruit of oaks, with a single large seed and a cup-like base".
According to an excellent book about the use of plants by animals of the U.S., "American Wildlife and Plants, A Guide to Wildlife Food Habits" by Alexander C. Martin, H. S. Zim and A. L. Nelson, "Oak trees are of major importance to both man and wildlife, and acorns are at a position at, or very near the top of the wildlife food list. Acorns provide a good and abundantly available staple - the staff of life for many wildlife species. The acorns of the white oak species are more palatable to wildlife just as they were preferred by the Indians and early settlers." The red oaks are much higher in tannins which makes their acorns bitter.
I don't know where this illustration of different seeds types "Common Types of Fruits and Seeds" came from, but we have used it for years with students and at workshops. The acorn is in the upper left-hand corner. (If you know the origin of this illustration , please let me know so I can give the author and/or artist credit.) 
Bur Oak has very large acorns (Quercus macrocarpa) ["macro" = large "carpo" = Greek for fruit] and the Bur Oak tree's acorns are the largest we have at the ranch. This one is over an inch wide and if you count the cup which is deep and fringed with course hairs in the measure it is almost 2 inches wide . The leaves of the Bur Oak can be up to a foot long. The leaves in this picture were 6 to 9 inches long.
The Bur Oak acorns on the left are huge compared to the much smaller Spanish Oak (Quercus buckleyi) acorns on the right (note the quarter in the middle of the large acorns for size comparison). The white oak family to which the Bur oak belongs are sweet. Red oak acorns are high in tannins which give them a bitter taste. Native Americans made flour and tasty dishes with white oak acorns. Animals love them too.
Bur Oak Acorns are very popular with animals. These acorns have been partially eaten, probably by squirrels. J David planted Bur acorns in pots by the Greenhouse, and covered them so no acorn loving critters couldn't reach them. He underestimated the cunning of squirrels, and was dismayed when he returned to find that eight of the containers showed evidence of tampering. Two of them still had an acorn and in the others the acorns were missing. 
These beautiful dark brown acorns are on a Plateau Live Oak tree (Quercus fusiformis) and are up to an inch long. The large number of Plateau Live Oak trees in this area mean that their acorns are important food for turkeys, deer, jays, titmice and woodpeckers. It is also the host plant for the caterpillars of skippers, hairstreak and admiral butterflies.
Live Oaks can get to be very large. This one, along Miller Creek has a sign that says "Largest Oak Known in Blanco County". I have no idea whether it is a Plateau Live Oak which is the common oak here at the ranch, or if it is the Coastal Live Oak (Quercus virigiana). It is huge and I think very, very old.
These acorns were on the ground under a large Lacey Oak (Quercuw laceyi) tree. We have a grove of these beautiful oaks along Miller Creek. I didn't find any of the smaller trees with acorns on them, so I was unable to get a photograph of acorns in their cups on a tree.
The old Lacey Oak trees in the grove are beautiful, with light gray bark, and graceful arching branches.
The leaves of the Lacey Oak have a thin coat of a waxy substance on their surface that reduces water loss during dry spells, and also gives the tree a bluish tint which has earned this oak the common name "Blue Oak". There are many animals that love acorns and some actually hide them away for later in the year when food is less plentiful. Squirrels and other small mammals bury them, and then forget where they put them. Thus they plant a new crop of oaks, and some of the new plants will replace the old ones that are dying. The young sprouts are also food for other animals.
I found out from Scott Grote, who manages the deer and hunts, that the acorn crop this year is large enough that the White-tail deer aren't as interested in our corn feeders as they are in years when the acorn crop is smaller.
There are many cycles in nature, warm and cold, wet and dry. It would seem that a dry year like this past 12 months would produce a small crop of acorns. However when you consider that we had an unusually wet spring in 2007, the extra water probably gave the oaks the health they needed to make it through a dry year. When looking at the health of trees you look at what has happened in the last 2 or 3 years, not just the recent seasons.
Enjoy the fall. Leaves are changing color. Lots of plants produce their fruit in the fall. The air is often cool. It is one of my favorite times of the year.
Of course, I would love to see some rain!
I found out from Scott Grote, who manages the deer and hunts, that the acorn crop this year is large enough that the White-tail deer aren't as interested in our corn feeders as they are in years when the acorn crop is smaller.
There are many cycles in nature, warm and cold, wet and dry. It would seem that a dry year like this past 12 months would produce a small crop of acorns. However when you consider that we had an unusually wet spring in 2007, the extra water probably gave the oaks the health they needed to make it through a dry year. When looking at the health of trees you look at what has happened in the last 2 or 3 years, not just the recent seasons.
Enjoy the fall. Leaves are changing color. Lots of plants produce their fruit in the fall. The air is often cool. It is one of my favorite times of the year.
Of course, I would love to see some rain!
