What is a Plant Family?

Every plant is an individual being, yes. But as humans, we love to categorize life into taxonomies. She's not just my dog, Macey, she's a Golden ShitzaDoodle!

When it comes to most life, including plants, we identify individuals as belonging to a species. These are groups of individuals that can produce viable offspring and generally inhabit shared habitat and geography. (In plants, more so than animals, interbreeding across species is quite common, and thus our scientific definitions are constantly being bent into new shapes… Descartes is not happy!).

For now, we will accept that individuals are members of a species. Groups of several species with a common ancestor are called a Genus (plural: genera). In modern biology, all species are referred to with a latin binomial that includes both the genus and the species. The genus Canis includes Canis lupus (wolves) and Canis latrans (coyotes). The genus Rubus includes Rubus parviflorus (thimbleberry) and Rubus ideaus (red raspberry). Thus every time we name a plant with its scientific name, we are getting information about its closest relative species because they are in the same genus. Since modern plant taxonomy is based upon evolutionary theory, the connectedness between two species within a genus is based upon both of them having evolved from a common ancestor. Rubuses - such as Thimbleberries, Raspberries, Blackberries and Salmonberries - all originated from a single original Rubus that is the ancestor of all the Rubus species that inhabit Earth today. The plants within a single genus are more closely related to each other than they are to plants in other genera.

A plant Family is the next higher level of connection up the taxonomic ladder. Families are large groupings of species that have something in common. Visually, the members of a family can have huge morphologic differences, but some similar common features are still observable. Plants within a family are related, but not as closely related as plants within the same genus. Spearmint and Peppermint are in the same Mint genus. Spearmint and Oregano and Basil are in different genera of the same Mint Family, which also includes Sage and Rosemary and Lemon Balm and Thyme.

Evolutionary proximity is how long ago the two species diverged from one another, and determines how closely related they are today. Thus the different families of plants diverged from each other a longer time ago (in evolutionary history) than the different genera and species within a single family. The latter are more closely related.

A great example is the ROSACEAE - the Rose Family. It is a large family of plants that contains almost 5000 species in about 90 genera. It contains trees like apple, pear, plum, peach, apricot, cherry, hawthorn, and almond. It also contains shrubs like wild rose and ornamental roses. It also contains brambles and vines like the aforementioned blackberries and raspberries and the other Rubuses. And it contains creeping ground cover plants like strawberry. The fact that a 100’ tall wild cherry tree, an ornamental rose, and a creeping strawberry are all related might seem strange… they have a lot of differences!

The key features of the Rose family, and of most families, lies in the structure of their flowers. Typical Rose family species have five petals, numerous stamen (more than you can count), and a set of partially-united green bracts beneath the petals called the “floral cup.” These flowers also have an inferior ovary, meaning that it is located lower down than the attachment points of the other flower parts - not that it is less important! Note that the flower color is not a unifying feature. Now think of the individual flowers of a cherry tree, a blackberry, and a strawberry. Think also of wild roses (Ornamental roses have been bred to have many more than five petals) Think of plum blossoms. All of these flowers have the same structure and arrangement. Apple blossoms and pears. Hawthorn trees. The petal color varies, but the flower structure of most members of the family is the same. 

Some plant families have common features that aren’t flower parts. The Mint Family (Lamiaceae) are plants that do have similar flower structure, but many of them are herbaceous plants (non-woody) with opposite leaves and square stems in cross section. 

The Mustard family includes lots of garden vegetables like cabbage, kale, cauliflower, arugula, mustard greens, horseradish, collards, kohlrabi, and brussels sprouts. The typical Mustard family flower is shaped like a little cross, with four petals: two long and two short. In fact, these plants, which now called “brassicas” were once called “crucifers” or “cruciferous vegetables” because of that little cross shaped flower. A fascinating thing about some of these is that many (kale, kholrabi, broccoli, cauliflower and brussel sprouts) are actually all the same species! Each has been cultivated, or bred, for different features. It’s exactly the same way for dogs - despite all the different appearances, all dogs are one single species. 

For the herbalist, learning about plant families is really fun, because it helps us understand the relationship between plants, how they evolved from common ancestors, and how the common features within families sometimes includes the ability to make common medicinal constituents. Cottonwood trees and Aspen trees are both in the Willow family, and all of them make anti-inflammatory salicylates. If you know about working with Willow trees but find yourself in a region with lots of Aspen trees, knowing that they have medicinal similarities is very helpful. 

For anyone who is excited to learn more about plant families, I highly recommend Thomas Elpel’s wonderful book, Botany In A Day. The title is a little misleading… I have spent dozens of days consulting this well written book.