Legumes and loments, cypselas and capsules, samaras and schizocarps! Fruits (in the botanic sense) come in a wide variety of shapes and sizes, many of which can be found growing inconspicuously among the shady canopies of the Arboretum. These hot and hazy summer days mark a critical transition period, as fertilized flowers transform slowly and steadily into their final seed-bearing structures. Unwitting observers may hastily pass over the green, photosynthetic tissues of developing fruits hidden between layers of dense foliage, as they search out showier midsummer blooms or air conditioning. Those willing to pause and look closer will find an array of colors and textures – a preview of the autumn’s botanical cornucopia.
Fruit development begins with pollination–the movement of pollen from the stamen (male structures) to stigmas (female structures) of flowers–which typically occurs from early spring to mid-summer by means of wind, insects, or animals. Following pollination, a microscopic tube grows from the pollen grain down through the stigma into the ovary of the flower, where the miraculous process of double fertilization takes place. Two sperm cells descend through this tube, one merging with an egg cell in the ovule to form a zygote, and its brother entering the endosperm mother cell to form the endosperm. During development, these two structures–the zygote and the endosperm–will give rise to the embryo of the seed and the nutritive tissues used by this embryo. Double fertilization is an elegant evolutionary adaptation unique to angiosperms (flowering plants) and a few unusual gymnosperms.
As individual ovules develop into seeds, the structure of the original flower and the arrangement of its organs will determine the shape and size of the ripening fruit. Simple fruits develop from a single or compound ovary in a flower with only one pistil, and can be dry (as in the case of the beech, hickory and Kentucky coffeetree below) or fleshy (quince, sweetgum). Aggregate fruits develop from the merging of several separate ovaries in a single flower (magnolias and tulip tree, above). Multiple fruits are formed by the union of multiple fertilized flowers (planetree, below).
The timing of fruit development is largely dependent on the reproductive strategies of individual plants, their flowering dates, and the conditions of the growing season. Early-flowing species such as elms, maples, and birches depend on the wind for seed dispersal, and produce rapidly developing, diminutive fruits. Trees with larger, more robust fruits, such as chestnut, oaks, and walnuts, often take longer to develop, and are timed to coincide with the activity of the animals that disperse them.
The study of fruit phenology is economically and ecologically important. Developing and ripe fruit serves as a food source for mammals, birds, insects, and fungi, and is especially important for the diets of larger, complex organisms that tend to be picky eaters. Fruit makes up a substantial fraction of the human diet, and the timing of agricultural practices and pest management are closely linked to our understanding of the reproductive cycles of the plants on which we depend.
In the wake of a changing climate, we can expect greater variation in phenological timing to compensate for rising global temperatures and erratic weather patterns. While a mild winter and late frosts blessed New England with a notably lucrative syrup season in 2016, these same conditions destroyed 90-100% of the peach and plum crops and a hefty fraction of apple crops in the region. Luckily, there is still a multitude of developing fruit to see at the Arboretum, so come check out these midsummer marvels before they’re ripe!
If you’re interested in learning more about phenology, botany, or ecology, join the Arnold Aboretum Tree Spotters, a group of citizen scientists responsible for collecting data from 11 species throughout the Arboretum. Our next training session will take place on Sunday, September 18 from 12:30pm to 3:00pm at the Hunnewell Building. Click here to register. Read more about Tree Spotters in Silva [pdf].