HomeHelpIdentify living thingsTop down method
The Top Down Method is the most scientific approch. Starting at a point of confidence, you methodically select a subset of possibilities, until you have identified your specimen, or at least have narrowed it down to a few choices.
On this page: Binary keys, Tree of life


Binary keys

A binary key is a decision tree that ideally leads to the most confident result. At each point, you select from one of two opposite choices. They are typically represented by the same number, with the second choice sometimes being identified with the prime mark (an apostrophe). If the first choice is not true, then the second should be true. Be sure to read both choices carefully. If neither seem to be true, you may have taken a wrong turn. Below is an example key for separating species among the genus Cardamine in the mustard family, taken from Shinners & Mahler's Illustrated Flora of North Central Texas:
 1. Petioles of lower stem leaves ciliate, at least basally; stamens 4; basal leaves larger and more
    conspicuous than the relatively few stem leaves __________________________________________________ C. hirsuta
 1. Petioles of stem leaves glabrous; stamens 6; basal leaves and lower and middle stem leaves +- similar.
    2. Lateral leaflets narrow; terminal leaflet usually 1-4 mm wide, usually not conspicuously larger
       than laterals; nearly all leaflets distinct, not decurrent along rachis; stems glabrous throughout;
       widespread in e part of ne TX _________________________________________________________________ C. parviflora
    2. Lateral leaflets broad; terminal leaflet usually 4-17 mm wide; usually conspicuously larger than
       laterals; distal leaflets often slightly decurrent along rachis; stems hispid basally; known in nc
       TX only from Dallas Co. _______________________________________________________________________ C. pensylvanica
There are some challenges to using binary keys.
  1. The keys may assume you have collected the specimen, and refer to something not captured in your photos or notes, such as the roots of a plant.
  2. Keys typically use a lot of technical jargon. Make sure you have a good glossary.
  3. If starting the key at a high taxonomic rank, the size of the keys may be overwhelming.
  4. It may be difficult to locate a binary key for your specimen.
The resources at the end of this page list a number of references that contain binary keys.


Tree of life

Follow the Tree of Life. That is, start at a known rank in the taxonomic hierarchy, then place your specimen in the next lower rank. Here is an example of how following the Tree of Life might proceed:
  1. While visiting Collin County, you took many photos of an herb with many small white flowers. It's is definitely a flower, therefore it must be in class Magnoliopsida.
  2. The flower has 4 distinct petals, and 6 stamens. 2 of the stamens are shorter than the others. This is typical for the mustard family, Brassicaceae. Here's a tip. When trying to place a flower in a family, study these first: Asteraceae (compound flowers), Fabaceae (legumes), Brassicaceae (mustard), Apiaceae (carrot), and Lamiaceae (mint).
  3. Using your plant checklist for Collin County, you read descriptions for each genus in the mustard family that is located here. Based on the petal color, shape of the leaves, the shape of the fruit, and the hairless stems, you conclude that the only match is genus Cardamine. Note that this was easier said than done.
  4. Your checklist shows two plants in this genus that are known to occur in Collin County. One of them, C. hirsuta, has only 4 stamens, so that's not it. The other is C. parviflora. You do a web search to find a description and pictures of this species to double check and conclude that you have indeed found Sand Bittercress.
This is the approach taken on TexasNature.net. Organisms are arranged taxonomically. Slowly details are being added to help narrow down to the next lower rank in the hierarchy. In some cases there are filters at the top of the page that can be used to hide any items that do not match a field mark of your specimen. Examples of pages with filters: Magnoliopsida, Asteraceae.

Resources: