Casuarinaceae are odd-looking plants. They have the standard tree-ish arrangement of a main stem and branches, but that's were the ordinariness ends. There are leaves but they're reduced to tiny triangles. They no longer have a role in photosynthesis. Instead, that job has been taken up by the branchlets, which grow from the main branches.
Each branchlet is made of sections called articles that lie end to end. (I described this in the first post but didn't have a photo to go with it.) You can see the articles and minute leaves in this branchlet from the Tasmanian short-range endemic Allocasuarina crassa. (By the way, everyone should have one of these marvellously sculptural plants in the garden.)
Each article bears a series of ridges (phyllichina) and furrows. The furrows (covered here by fine white hairs) contain stomata, which are pores for the uptake of carbon dioxide used in photosynthesis. Furrows reduce water loss from stomata. Those in the Wet Tropics endemic Gymnostoma australianum (left) are shallow and open, whereas those in Casuarina and Allocasuarina, which live in drier areas, are deep and narrow.
The shape and number of the phyllichnia and their associated leaves are important in distinguishing between species. Here's a third species, Allocasuarina monilifera (another Tasmanian endemic) to give an idea of how this group can vary.
Of course, there are other key features, such as flower, cone and seed (samara) shape but you have to be there at the right time for these to be of great use. The branchlets are present all year round.
If you want to know more about Australian Casuarinaceae, the relevant section in the Flora of Australia is available on line.