The process of identification
Although correctly identifying an organism is not always easy, the process of identification is actually quite straightforward. The most important elements are careful observation, attention to detail and - the most difficult - correct interpretation of what you see. Some guidelines are given below to help you.
All plants have features (called characters) which help to identify them. Some are very obvious, some are less obvious. Careful observation will help you avoid errors.
It is possible to mistake the parts of a plant. For example, the leaflets of a compound leaf can be mistaken for leaves and coloured sepals can resemble petals, leading to a wrong interpretation. You can check which parts are which by relating them to other features. Thus, leaflets are joined to a leaf stalk, not directly to a twig while sepals (whatever their colour) form the outermost whorl of flower parts.
The absence of a character is often important for identification. But how do you know that the plant never has this character? It may be present but not easily observable (the number of seeds inside a fruit); not yet developed (fissures in old bark); or have developed and already disappeared (bud scales that fall very early). This is probably the most difficult aspect of identification. Fortunately, it is also rare. In most cases, if you cannot see a character, it really is absent but careful observation reduces the risk of error.
Some characters vary more than others. Several factors are involved, including age, ecological or local conditions, the season etc. For example, leaves can be very variable, especially in size. Leaves produced low down on the stem (early in the season) may be smaller than those higher up. Plants growing in rich soil may have larger and more lush foliage than those in thin or dry soil. Leaves produced in shade may differ in size and colour to those produced in full sun and leaves produced on young trees may be an entirely different shape to those on old trees of the same species.
Sizes are therefore often given as a range e.g. 5-10 cm. When determining the size of an organ or even a whole plant, do not simply chose the largest or smallest example. It is more useful to measure (say) 10 of them and take the average. Even so, it is not unusual to find an example that extends beyond the range.
Other characters are conservative - that is, they show very little variation whatever the conditions. For plants these include flower shape and size, fruit type and size, hair type (but not hair density). Conservative characters are therefore more reliable for identification and identification tools often use them more than variable characters.
Sometimes the parts you need to examine are no longer attached to the plant. This is especially true of trees, where sometimes the only fruits, leaves or flowers available are those that have fallen on the ground. Check that these unattached parts are from the plant you are examining and not from a different one nearby.
Just because two specimens look the same at a quick glance does not mean they are the same. Many species, especially closely related ones, differ only in minor ways. Check each specimen to ensure it has all the relevant characters and that your identification is correct.
Additional evidence to confirm your identification can often be inferred. For example, the arrangement of fruits can tell you how the flowers that preceded them were arranged. Distribution and ecology can also be helpful. You would not expect to find a plant known only from N. Europe in the Mediterranean. Similarly, a plant adapted to mountain cliffs is unlikely to occur on lowland plains and a woodland species is unlikely in open meadows. But such evidence should only be used to help confirm an identification rather than forming the sole basis of it. In recent years, plants of coastal salt marshes were found growing along the sides of inland roads. This apparently 'wrong' distribution was due to regular salting of the roads in winter, providing the conditions these plants need to thrive.
Smelling plants is acceptable as the scent of both foliage and flowers can be a useful clue to identity but tasting plants should be avoided - some are poisonous!
Most keys offer clear choices at each step. If you cannot decide which choice to take in a key, try going down each route offered. The correct one will soon become clear. If neither seems correct, you have taken a wrong choice earlier on in the key, so retrace your steps or start again.
This is perhaps the most useful character of all. Becoming familiar with a group of organisms will increase the frequency and accuracy of your identifications.
Identification to species level may not be possible, but you may be able to identify the organisms to genus as this usually requires fewer characters.
Don't expect always to achieve an identification - the characters you need may not all be present at the same time.
Sometimes the identification turns out to be wildly wrong. Don't worry - it happens even to experts!