Sometimes it is easy to see the family resemblance among species of the same genus. Take
the genus Viola. The similarities between pansies, violets, and johnny-jump-ups are
easy to see. For example, they have similar flowers with five petals, with the lower petal
the largest and the other four 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 tree euphorbias; 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 euphorbias 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.
Understanding the details of the complex systems used to
classify plants—and all living things—isn’t necessary, but gardeners do
benefit from having some familiarity with a plant’s place in the system of
classification. Members of a genus often share common characteristics, such as soil and
light preferences. For example, willows (genus Salix) generally prefer moist soils.
Knowing this, you might begin your search for a streambank stabilizer by looking at the
various Salix species. (Of course the euphorbias illustrate that this resemblance
among species within a genus isn’t absolute!)
And, conversely, knowing a plant’s botanical name can
sometimes help you determine its particular requirements. Suppose you receive a potted
plant as a gift, and all the label gives is its name, Cornus officinalis.You’re
wondering what type of plant it is, and what conditions it needs.You check your reference
books, and see that it’s a dogwood, but can’t find that particular species.
However, in the process you notice that all the other Cornus species mentioned are
large shrubs or small trees that prefer dappled shade and moist, acidic soils. So, you
make a guess that these things also apply to your new plant, and in this case you would be
right. (Though it would be a good idea to call a local nursery to doublecheck this.)
