In the autumn the acorns are ripe, and falling. Some trees will show acorns of two sizes, half-grown ones on the new shoots, and full-sized ones on the bare twigs, just back of the new shoots.
This peculiarity divides the oak family into two great groups. One group is composed of trees which have light-coloured bark, bear a crop every year, and in winter are bare of fruit. This is known as the White Oak Group. Its leaves have rounded margin lobes which do not end in sharp points, as many of the lobes of oak leaves do.
All of the oaks whose leaves have pointed, spiny lobes on their margin belong to the Black Oak Group. The bark of these trees is usually dark-coloured. The acorns require two years of growth. For this reason, there are half-grown acorns on the tree all winter, waiting for the second summer to bring them to maturity. Every autumn the acorns which are ripe are found on the twigs just back of the leafy shoots, which grew during the past summer. These acorns have completed their second year of growth.
When we hear any one speak of annual-fruited and biennial-fruited oaks, we know that the White Oak and Black Oak Groups are meant. If you see an oak tree whose leaves are cut into sharp pointed lobes, you will find acorns of two sizes on its twigs. If you look across the fence and see a pale-barked oak with finger-lobed leaves, and not a spiny point on their margins, you will know that acorns of but one size will be found. Fix these three points in mind. Then study all the oak trees you can find.
Trees of the White Oak Group have:
1. Rounded lobes on their leaf margins.
2. Acorns ripe in a single season.
3. Pale-coloured bark.
Trees of the Black Oak Group have:
1. Spiny-pointed lobes on their leaves.