From Seed to Seed:
Plant Science for K-8 Educators

Family Resemblance

Sometimes it is easy to see the family resemblance among species of the same genus. Take the genus Viola. The similarities between a pansy, a violet, and a Johnny-jump-up are easy to see. For example, they have similar flowers with five petals, with the lower petal being the largest and the other four petals forming two dissimilar pairs.

On the other extreme, the genus Euphorbia contains species with vastly different forms and habitats. This genus contains the familiar poinsettia, a non-hardy shrub native to Mexico, as well as the crown-of-thorns, a native of Madagascar with needle-like spines commonly grown as a houseplant. Other genus members include snow-on-the-mountain (Euphorbia marginata), a vigorous, bushy annual plant native to the prairies from Minnesota to Colorado, as well as a number of garden spurges such as gopher spurge (Euphorbia lathyrus). Remarkably, there are even trees in this genus; Euphorbia canariensis, native to the Canary Islands, has a cactus like appearance and grows to a height of 40 feet.

How can such diverse plants be so closely related?

Classification of higher plants is primarily based on reproductive structures, and in this respect, all members of the genus Euphorbia are similar. We won't go into the specifics here, but suffice it to say that the flowers of your holiday poinsettia (the yellow-green structures in the center of the brightly colored bracts) have a structure that is shared by all euphorbias.

It is not necessary for you or your students to understand the details of the complex systems used to classify plants, but it can be beneficial to have some familiarity with a plant's place in the system of classification. Take your students out to your schoolyard garden or to a wildflower patch and ask them to find pairs of plants that may be closely related. What did they base their decision on? Flower color? Size? Shape? Leaf size or shape? Alternatively, if you do not have access to an outdoor spot, you can bring a variety of cut flowers into the classroom and have students pair up likely relatives.