The Bentham and Hooker Planting Sequence
In the late 1870s, when Charles S. Sargent and Frederick Law Olmsted were confronted with the question of how to arrange the collection of woody plants envisioned for the new Arnold Arboretum, their goal was to show relationships among the various genera and families of plants by placing related groups in close juxtaposition, thereby enhancing the educational value of the collection and permitting easy botanical comparison. The system they followed—the natural classification system of Bentham and Hooker—was widely accepted by British and American botanists of the time, having been proposed by George Bentham (1800-1884) and Sir Joseph Dalton Hooker (1817-1911) in their monumental three-volume publication, Genera Plantarum, published in Latin between the years of 1862 and 1883. Bentham and Hooker, leading English systematists of the day, conducted their botanical investigations at the Royal Botanic Gardens at Kew, where Sir Joseph had succeeded his father, Sir William Jackson Hooker, as director. Genera Plantarum included precise descriptions of all the genera of seed plants then known to science and grouped them based on overall similarity into "cohorts" (equivalent to today's plant families). The cohorts were similarly grouped into orders, classes, and finally, into the three major subdivisions of woody plantsdicots, gymnosperms, and monocots.
According to the Bentham and Hooker system, plants producing flowers with separate or free floral parts were less advanced, or more primitive, than those producing flowers with united parts. The magnolia family (Magnoliaceae) was considered one of the most primitive groups; consequently, the magnolias and their close relatives, tulip trees, were established near the Museum building (now the Hunnewell building), where the planting sequence begins. Progressing through the collection from the Arborway Gate, the visitor encounters plant families of increasing morphological complexity, as evidenced by such features as reduction, connation, and loss of floral parts. Bentham and Hooker classified the conifers in a distinct group between the dicots and the monocots; since there are few woody monocots, the conifers came last in the planting sequence and were sited along Bussey Brook at the end of the original road and pathway network. And so, the pre-existing grove of native hemlocks found its place on Hemlock Hill.
Very few exceptions were made to the overall planting scheme. In some cases, ecological requirements were a constraining factor; the willows, for example, were planted adjacent to the low-lying ground of the meadow rather than in their position in the Bentham and Hooker sequence. Another exception was made for ericaceous shrubs and flowering dogwoods, which were planted in various locations for aesthetic appeal and to knit the collection together into a harmonious whole.
Over the years the planting scheme chosen by Sargent and Olmsted has not always been strictly followed. In the early 1980s, however, an effort to restore the original layout of the collections was initiated, and today the major groupings in the collection once again adhere to the historic Bentham and Hooker classification.
Adapted from "The Bentham and Hooker Planting Sequence in the Arnold Arboretum," by Stephen S. Spongberg, in CURATING THE LIVING COLLECTIONS, Arnoldia (1989) Volume 49, Number 1. Copies are available for purchase.