In January, during a trip to the Arnold Arboretum, I picked up a tree branch and brought it home. The removal of living plant material from the Arboretum is strictly prohibited, and I am not normally one to thwart the rules, but this branch had been pruned and was destined for the compost pile. Back in my hot, dry apartment, I plopped the rescued branch in some water and left it near a window, hoping to coax some flowers from its large, showy buds, and bring some spring into our home.
The branch had come from a magnolia tree, specifically a cultivar called Magnolia ‘Elizabeth’, growing near the entrance of the Arboretum. A blooming magnolia is a welcome and familiar sight for a New Englander. In Boston, magnolias, along with dogwoods, cherries and plums, are harbingers of spring. The unfolding of their large and luxuriant blossoms accompany a collective sigh of relief at winter’s end, and signal a flurry of new activity after the stuffy indoor months.
The intrigue extends beyond large and fragrant flowers. Magnoliaceae is an ancient family and among the oldest living angiosperms, with fossils dating back between 36 and 58 million years. Relatively unchanged since its primitive beginning, a blooming magnolia affords us a glimpse at one of the earliest flowers to develop on earth.
Above, the dormant flower bud of Magnolia ‘Elizabeth’ is covered with a stipule, or modified leaf, that protects it through winter. Below it the flower bud, smaller vegetative accessory buds will develop into foliage.
During late summer, magnolias begin developing buds for next season. By fall, they have finished preparing these buds, which are large and very noticeable. Covered in fine hairs called trichromes, magnolia buds are well adapted to the frigid temperatures and drying winds of winter. The hairs prevent frost from contacting and damaging the outer layers of the bud. They break the flow of air traveling across the bud’s surface, increasing the humidly and protecting it from drying out. The hairs also provide a degree of resistance to UV radiation from the winter sun.
All of these evolved adaptations help dormant buds survive winter, eliminating the need to invest time and energy into creating these reproductive organs in early spring. Trees with dormant buds can flower as soon as warm conditions and extended daylight trigger certain chemical signals, and this expediency affords them a serious advantage over plants that require more time to begin their reproductive cycles. Of course, the possibility of a late winter freeze adds an element of risk, as an unanticipated frost can quickly destroy venerable flowers and foliage.
These vegetative buds, which will produce foliage after the tree has flowered, are less significant but no less elegant. On this branch, we can see two alternate axillary buds along its length, and a pseudoterminal bud at the apex. All of these buds developed in the axils of last years leaves, where the petioles (or leaf stalks) connected to the branch. The traces of last years foliage can be seen in the triangular scars under each bud, where the vasculature of the leaves connected to the branch. The white dots along the branch, called lenticels, are small organs that allow gas exchange between inner tissues and the outside environment.
Once buds have formed in autumn, they contain all the necessary primordia (differentiated tissues in their earliest recognizable stages of development) to later create fully formed flowers. With the advent of spring’s warmer temperatures and longer days come the chemical signals that cause a bud to begin swelling in preparation for flowering. During this time, rapid cell division leads to the formation of the flower’s anatomy, all packaged within the expanding bud. As the bud swells, the protective outer layer (in the case of magnolias, a single adapted stipule from last season) breaks open, exposing the newest sepals, petals and leaves.
In the second image, a cross section of the bud’s base reveals the vasculature that connected the bud to the stem, transporting water, nutrients and hormones to the developing tissue. I dissected this bud on February 13th, 21 days after I first began observing the branch in my apartment.
Peeling away the outer protective layer reveals partially developed organs, still vulnerable.
A longitudinal bisection reveals young organs in the process of becoming fully formed flower parts. Large anthers, or male reproductive parts that will bare pollen, can be seen with red bases. They are grouped around the central gynoecium, a whorl of pistils, or female reproductive parts, that will be pollinated.
In most angiosperms, the petals and sepals of the flower are easily distinguishable. In magnolias, these two outer organs are virtually the same, and are thus referred to collectively as tepals. In the image above, the premature tepals have an orange tinge due to rapid oxidation.
By February 18th, the stipule and outer scales have peeled back significantly and the silky white tepals are beginning to emerge. A small leaf can also be seen emerging from the bud.
One day later, the flower is beginning to fully open, and looks noticeably different every hour.
As I mentioned earlier, magnolias evolved well in advance of bees, and depended on bulky and significantly less graceful (and more ravenous) beetles for pollination. The large, tough tepals of the flower helped it survive a great deal of abuse during the process.
By the afternoon, the flower had fully open revealing the intricate anatomy that had developed over the previous weeks. The image above was made 26 days after I brought the branch inside and exposed it to temperatures around 68F during the day and 62F at night.
In this image, we can see the venation of a bud scale, similar to blood vessels in a human, that allowed water and nutrients to feed this tissue as it developed and eventually fell away.
The inflorescence of the magnolia is a truly magnificent example of Nature’s design. Anthers, arranged in a spiral, can be seen encircling a group of interlaced pistils in the center. The robustness and compactness of this arrangement allows large insects to traverse from male to female parts, pollinating the flower as they go. The pollen itself, rich in protein, is incredibly fragrant, enticing to beetles and humans alike. Once spent, the anthers will fall away, and inside the swollen ovaries, fruit will being to form.
I made this final image of a Sweetbay Magnolia (Magnolia virginiana) seed pod late last fall. The pod itself is nearly four inches long, and before it had been picked clean, would have contained a number of bright red seeds dangling from each open “pouch”. Each of these individual seed containers was formed by a fertilized ovary in the center of the flower, which finally broke open and bore its seed to the earth or a hungry passerby. The remaining pod is a strange, twisted relic of the complex geometry and fleeting sweetness of the magnolia blossom.
Coming soon to a tree near you!
Thanks for reading,