This is a note I made in summer of 2013. Post here as archive.
Branches appearing flattened, the leaves strongly appressed to stems primarily in one plane or at least squarrish in cross section.
Diphasiastrum tristachyum Branches appearing somewhat squarrish, rather than broadly flattened; often conspicuously blue-green in color; all leaves about the same size; ultimate branches 1-2.2 mm wide.
Branches strongly flattened; green; leaves on the flat underside of branches shorter than the other leaves; ultimate branches 1.8-4 mm wide.
Diphasiastrum complanatum: Branches with conspicuous constrictions (narrowings); lacking sterile tips on the strobili.
Diphasiastrum digitatum: Branches without conspicuous constrictions; sterile tips present on many strobili.
White oak group
Quercus alba: Glabrous at underside of leaves.
Quercus macrocarpa: underside of leaves has pubescence, only one stellera type hairs.
Quercus bicolor: underside tomentosa, two types of hairs.
Red oak group
Quercus rubra: the deepest sinuses extending 45-70% of the distance to the midvein; the petioles and green branchlets sparsely hairy or glabrous when the leaves are just expanding. Acorns.
The petioles and green branchlets densely hairy when the leaves are just expanding. the deepest sinuses extending 65-90% of the distance to the midvein.
Quercus velutina: Lower surface of leaves more or less pubescent; buds hoary-tomentose; cup scales of acorn silky-pubescent, spreading and squarish at the tip; lower dead branches not persistent.
Quercus ellipsoidalis: Lower surface of leaves glabrous or nearly so; buds glabrous, lustrous, only slightly angled in cross section; stubs of dead branches persistent on lower trunk.
Toxicodendron radicans: Tall branching shrub often > 2m tall, or vine climbing with aerial roots; perioles moderately to densely hairy; leaflets ovate, usually more than 1.5 times as long as wide.
Toxicodendron rydbergii: Low shrubs with few if any branches and usually < 1m tall, not producing aerial roots; petioles glabrous or sparsely hairy; leaflet broadly ovate, usually less than 1.5 times as long as wide.
- Only Cinnamon fern has the pale tan tufts of wool at the base of the pinnae.