"In Peter Bernhardt's eloquent, accurate and literary book, floral biology . . . is explained to inform and entertain everyone, from curious scientists to amateur natural historians."—Peter G. Kevan, Nature

An excerpt from
The Rose's Kiss
A Natural History of Flowers
by Peter Bernhardt

Chapter 4
When to Bloom

"Just at this time the hundred years had nearly come to a close, and the day at last arrived for Briar Rose to be awakened from her long sleep. On this very day the prince started on his enterprise and on reaching the hedge of thorns, what was his surprise to find it covered with delicately-beautiful flowers."
—Jacob and Wilhelm Grimm, "The Sleeping Beauty in the Wood"

A tropical tree drops all its leaves as the season turns hot and dry but quickly reclothes itself in thick clusters of flowers. A snow crocus pushes yellowish buds through defrosting soil until they reach the weak light of a February morning. Although these displays occur on different continents, both will enjoy human admirers.

Most people welcome the sight of flowering plants after a drab, uncomfortable season and are willing to extend the display by playing with plant heredity. For example, some collectors prefer the older breeds of roses, extolling their hardiness, strong scents, and fine colors. Unfortunately, most of these old forms stay in flower for just two weeks each year. Go to your local nursery and you'll find it's the finicky modern breeds that sell out because they promise "repeating roses" from late spring through autumn.

Where are flowers made in a plant? They seem to rise up magically out of the bare earth or, more often, appear to emerge out of formless masses of stems and leaves.

Branches, leaves, and flowers all derive from the same microscopic source. They are manufactured by the reduced tip of a stem. This tip is hard to see with the naked eye, as it is often covered by developing leaves during the growth season or by bud scales during periods of dormancy. The tip is properly known as the shoot meristem, a name derived from the Greek word merizein, "to divide." By cutting thin longitudinal sections of a shoot and staining them, botanists can locate the regions of most intensive cell division, since meristems are the dominant centers of growth in plant bodies. Meristems are composed of rapidly dividing cells called initials. The shoot tip of a flowering plant is always made of layered initials.

The outermost initials form the tunica. Tunica initials are small and, as their name suggests, well organized into tight but orderly layers, like the clothes covering your body. Most tunica initials divide to make plant skin and all the hairs, glands, and bud scales on the outer surface of the plant. Below the tunica is a mass of packed, swollen initials that make up the body, or corpus, of the shoot. Corpus initials divide at various angles and planes, adding bulk to the inner parts of the plant by forming pith and vein tissues. Working together, the tunica and corpus increase the length of the primary stem and manufacture all leaves and new lateral buds. A lateral bud usually appears at the base of each new leaf. This new bud waits patiently for the central stem to age until it is permitted to sprout by itself, forming its own leafy lateral branch.

The tunica-corpus spends most of its active life adding foliage and length to the stem, until it receives signals to change production. When these signals arrive, the cells in the shoot narrow and stretch so much that leaves and lateral buds are unable to form. It's then time for meristem initials to make flowers, flower stalks, and those flat, protective enveloping structures (bracts) that appear at the base of most flower buds or their stalks.

This transition from leafy stem to flowering branch is usually permanent, but there are two developmental extremes. Sex and death are linked in many of the plants sold as bedding annuals. Alyssums (Alyssum), marigolds (Tagetes), tufted pansies (Viola cornuta), and zinnias (Zinnia elegans) convert all their shoots on every stem into flowering branches as they age over a single growing season. Unable to make leafy stems after reproducing, annuals die of old age even if the frost doesn't kill them. The life cycle of a zinnia, then, is similar in some ways to the lives of most insects or Atlantic salmon. That's why the only way to keep your favorite potted coleus (Solenostemon rotundifolius) alive indefinitely is to pick out every flowering stalk as soon as flower buds are visible. By removing a flowering stalk, you trigger the activity of lateral buds lower down on the stem, encouraging them to sprout and make new leafy stems.

At the other end of the spectrum are some Australian members of the myrtle family (Myrtaceae), especially shrubs such as bottlebrushes (Callistemon) and honey myrtles (Kunzea). In these plants, a stem shoot converts itself into a flowering stalk, but the flowering stalk "reconverts" into a leafy stem shoot as soon as its flowers form fruits. No one is really sure how they do this. However, since these evergreen bushes usually bloom only once a year, the age of a bottlebrush branch can be determined by counting the number of hard fruit clusters on it, since each cluster represents one year of reproductive effort.

Most perennial herbs, shrubs, trees, and woody vines prefer an intermediate path to flowering. Secondary branches and twigs flower, fruit, and die, but primary stems and trunks are preserved to increase growth and extend the plant's life span. Many wildflowers return year after year because they keep their primary stems underground and poke only their seasonal secondary stems up into the air. The primary stems form rounded bulbs, pointed corms, lumpy tubers, or jointed, creeping structures known as rhizomes.

What signals "convince" a stem to make flowers? Although different plants grow at very different rates, we recognize that our local vegetation experiences peaks when the flowers of many different species bloom together. These annual miracles are so dependable that some people travel abroad just to enjoy the flowers of rural England in April, those of Mexico at Christmas, or those of South Africa in September.

Supernatural explanations for this phenomenon dominated imaginations until people began to experiment seriously with plants. Cultures recognizing many gods or spirits assigned the annual flower show to some minor deity, often female. The ancient Greeks said that Princess Chloris became the goddess of flowers after the death of her mortal family. The Romans worshiped the nymph Flora, who exhaled petals and left flowers in her footprints. Her lively festival began on April 28 and continued for six days while celebrants played games, exchanged floral crowns, and invited prostitutes to undress in public. The Chinese Fairy-of-a-Hundred-Flowers was a strict bureaucrat who made sure that each blossom didn't open until its appropriate season. Japanese parents once told their children of Konohana-sakuya Hime (Princess Blossoms of the Trees), the lady who causes trees to bloom. A tribe of Koori people in Australia believed that flowers were a gift from the spirit of a powerful male shaman. Byamee received messages from the bees before he made the east wind blow the rain down the mountain to soften the hard ground and bring forth flowers that his bees could visit to make honey.

During the twentieth century, scientists began to understand how environmental cues stimulate flowering. Clearly, plants do not respond to external cues as quickly as animals do. Plants tend to require a rhythmic, repetitive cycle over a period of weeks or months before they make buds or become ready to open them. This period, in which a plant's physiology must follow the rhythm of the cue, is known as entrainment. Eventually, flower development becomes synchronized with one or more cues that are repeated every twenty-four hours. Plants that entrain to the same cue or cues show peak bloom around the same season, regardless of species.

Cues encourage plants to make new hormones and suppress old ones. In some plants, hormones may move to and from the meristem through the cells that link up to form the young veins. Since the early 1930s, plant physiologists have tried to isolate the hormone that turns leafy shoots into flowering stalks. They call this mystery chemical anthesin, but I don't think they'll ever isolate it. Research suggests that different plants require different hormone mixes to bloom. Some plants need gibberellins, a class of hormones that make cell walls stretch. Others are content with ethylene, surely one of the shortest compounds made by a plant (CH2=CH2). Ethylene is such a lightweight molecule that it may escape from one plant only to be absorbed by another. For example, ripe apples give off ethylene, and an old but effective trick to make bromeliads bloom is to lay a piece of apple on the plant and cover them with a paper bag for a short time.

In the Northern Hemisphere, most research on flowering has concentrated on the cue of light and dark cycles. Plants that respond directly to these cycles fall into two broad categories. Short-day plants form flower buds when synchronized to a cycle of reduced critical day length. Most short-day plants bloom in early spring or fall, often requiring fewer than fourteen hours of light each day. Cockleburs (Xanthium strumarium), strawberries (Fragaria), primroses (Primula), a type of morning glory (Ipomoea hederacea), and some breeds of tobacco (Nicotiana) are among the best studied of the short-day species.

Long-day plants require entrainment periods opposite those of their short-day cousins. They need daylight cycles that exceed some critical interval, usually blooming during summer. Lettuce (Lactuca), most of the older breeds of potato (Solanum tuberosum), barley (Hordeum vulgare), and spinach (Spinacia oleracea) are among the best-known of the long-day plants.

Light cues tend to explain some odd features of agriculture and horticulture in the Northern Hemisphere. For example, few people who live in farming regions where the summer season is brief ever see the bluish purple flowers of potatoes. That's because the wild ancestor of the domesticated potato evolved in the Andean highlands of South America, where longer days are the rule.

This also explains the proliferation of greenhouses with white-painted panes that shut out much of the sunlight. Within these greenhouses are tropical plants receiving fixed cycles of lamplight so they will bloom for winter sales. The beloved poinsettia (Euphorbia pulcherrima) is actually a short-day plant, but it won't survive long outdoors in a November freeze. It requires the same critical intervals of light its parents receive in Mexico, so the potted plants must be placed on an artificial cycle to produce Christmas flowers.

Green leaves "measure" the cycles of light and day. This is to be expected, since it is the green leaves that capture light energy to make sugars. During the 1930s, the humble cocklebur was subjected to a series of laboratory tests. Cockleburs are so tough that they will continue to live even if they are completely defoliated. However, a cocklebur can't flower if its leaves are removed during the critical cycle of short days. Scientists snipped pieces off cockleburs' leaves and learned that only one-eighth of a mature leaf on a stem was needed to receive the light cue and trigger the flowering process.

The response of leaves to light cycles seems to be very localized. Kalanchoe blossfeldiana is a succulent from Madagascar popular as a houseplant because it produces reddish flowers in winter. You can take a single plant and expose its leaves to two separate light cycles by shutting different branches of the plant in separate light boxes. Flowering stems will develop only near those leaves that receive a short-day cycle. Flowers won't form on shoots whose leaves are exposed to a long-day cycle.

Many plants require two periods of entrainment, and that often means exposure to two different cues at two different times. The first cue induces the development of flower buds, while the second cue encourages the buds to swell and open. Many trees and shrubs of the Northern Hemisphere respond to light cycles, producing their flower buds by late summer or autumn. The buds of lilacs (Syringa), ornamental quinces (Chaenomeles), and breeds of Asian magnolia are easy to find on autumn branches once the leaves have fallen, but they remain as buds until spring. Having produced their buds, the plants require entrainment by a cycle of low temperatures. After a couple of months of cold weather, they will be ready to swell and pop as days grow warmer.

There are problems with this double cycle when plants are grown outside their natural distribution. In the St. Louis area, fruit crops are more likely to be devastated by mild seasons than by frigid weather. Buds start expanding during extended thaws in early March, but opened flowers are slaughtered if hard frosts return unexpectedly in April. Cinema fans may remember the Sardinian peasants in the beautiful film Padro Padrone, whose hopes for a better life are crushed after a mild winter. They lose a year's olive crop when the early flowers are killed by a freeze so hard it turns a pitcher of goat's milk into sorbet.

Temperature cues are all some plants need to stimulate flowering. This helps explain how bulbs and tubers ready their flowers, since these underground stems lack aerial leaves for most of the year. Daffodils (Narcissus), tulips (Tulipa), hyacinths (Hyacinthus), and bulb iris (Iris reticulata) are descended from wild plants native to true Mediterranean zones. These regions experience hot, dry summers and cool, wet winters. The commercial bulbs gardeners purchase each autumn have already received their first hot cue. In some cases, the bulbs were dug up after the spring leaves withered and stored in a dark, dry warehouse for a week or more at 80-85 degrees Fahrenheit. That's all that is needed to simulate the natural conditions of clay and sandy soils baking under a Mediterranean sun. Cut a heat-treated bulb in half lengthwise and you'll find embryonic flower buds embedded inside.

Planted in the moist autumn soil of a temperate garden, a bulb begins to grow roots at its base. Although some grape hyacinths (Muscari) and hardy cyclamens (Cyclamen) will poke tough leaves above the ground by late autumn, a bulb's flowers won't appear until after a cold cue. Gardeners must remember that northern winters are so severe that imported Mediterranean bulbs often delay flowering for weeks or months after their usual period of cold entrainment has ended. In fact, most garden bulbs and tubers are bred from wild ancestors that bloom best if they are entombed in a frigid dirt for less than twelve weeks. Take a winter holiday in a Mediterranean zone and you may see that some native species complete their flowering before spring. In Israel, for example, some Crocus species, the mandrake (Mandragora officinarum) and the red windflower (Anemone coronaria), are flowers of mid- to late winter. The native vegetation of southern California also evolved in a Mediterranean climate. Guides to California wildflowers mention that some mariposa lilies (Calochortus), blue dicks (Dichelostemma), and mission bells (Fritillaria) start flowering as early as February.

What about the vast majority of plants inhabiting the Tropics? Since the earth tilts on its axis, both day length and temperature should become less important as flowering cues as we approach the equator. This is, in fact, the case. The plants of equatorial deserts, savannas, and monsoon forests appear to use water as a cue. Their entrainment reflects annual changes in the moisture level of thin soils.

It may seem like a contradiction, but some trees wait for the beginning or end of the dry season to bloom. Heavy, warm rains are believed to promote fungus infestations and other flower diseases while suppressing the movements of pollinators such as flying insects, birds, and bats. Since flowers are short-lived and lack breathing pores on their petals and sex organs, they don't waste as much of a tree's stored supply of water as one might think. Therefore, the dry season in lowland coastal forests from Mexico south to Panama is often the most colorful time of year. Trees belonging to the bean (Fabaceae), trumpet vine (Bignoniaceae), dogbane (Apocynaceae), cola nut (Sterculiaceae), and kapok (Bombacaceae) families decorate the dry countryside in rich shades of yellow, ivory, purple, and hot red. Wet and dry cycles also entrain some shrubs and many smaller plants such as bromeliads and begonias. In western Africa, though, most orchid species ignore the expected pattern and flower during the wet season.

Some temperature cues can be expected even at the equator. Mountain ranges host many tropical plants at different altitudes. The higher the altitude, the more the temperature will fluctuate over the year, and many montane plants respond to the most subtle cues of low night temperature. Among these plants, the orchids receive the most attention, as the corsage industry demands precise results. Tropical slippers (Paphiopedilum), moth orchids (Phalaenopsis), Cooktown orchids (Dendrobium), and miltonias (Miltonia) are just a few of the orchids that refuse to bloom without a cycle of cool nights.

Understandably, the flowering periods of trees in lowland equatorial rain forests have been less easy to predict, since the cycles of light, temperature, and rainfall seem so monotonous. Some trees appear to stay in flower almost all year once they've reached a certain size and girth. This is common in some members of the Brazil nut (Lecythidaceae) and cocoplum (Chrysobalanaceae) families. A feature of such trees is that they appear to stagger their buds, opening only a few each day to replace those that die or mature into fruit. They dribble their flowers over the year instead of massing them into one grand show.

In my opinion, the most surprising and dramatic of all flower shows belongs to vegetation that has evolved with fire. We usually think of fire as one of the greatest of plant enemies, and in most environments, that is true. However, plants belonging primarily to Mediterranean woods and shrublands are often exceptions to this rule, as brush fires are incorporated into their long-term cycle of regeneration and reproduction. Before human beings altered the vegetation of southern Australia, southern California, southern Africa, and the Mediterranean basin, lightning strikes made dry tinder go up in flames every few years.

Yes, a Mediterranean shrubland appears violated after a summer fire, but the majority of plants are probably still alive. Certain shrubs and trees will sprout from their stumps or rise again from "emergency" rootlike bodies known as lignotubers. The heat also cracks the coats of seeds buried a few inches down in dirt and duff, making them receptive to rainwater and germination. The cleansing fire frees bulbs and tuberous plants from unequal competition with overshadowing bushes. Within a few months, autumn and winter rains leach minerals from the burnt wood and ash, returning these essential nutrients to living roots in the soil. By spring, new growth and flowering become intense, often with rare species that may not have been seen for decades blooming in lush colonies. In Australia in particular, the spring following a bush fire provides the best excuse to take a "sick day" and search for unusual lilies, carnivorous sundews (Drosera), and rare members of the daisy family.

For most Australians, the real sign that fires have stimulated optimal flowering is seen in the twenty-eight species of grass tree (Xanthorrhoea). Grass trees look as if they are wearing hula skirts made of grass blades, but they are really asparagus-like plants thought to be related to our potted snake plants (Sansevieria). In the absence of fire, the largest species of grass tree tend to bloom rather sluggishly and sporadically from late spring to autumn. The spring after a fire, though, each mature tree will have lost most of its "skirt" and its trunk may wear clots of hard, red resin. The center of the burnt plant grows a thick, erect stalk resembling a giant asparagus spear covered with black, bristly bracts. In some species, the flowering branch may be more than six feet tall, with some parts of the spear thicker than a man's arm. Grass trees are among the plants that hurry into bloom on exposure to ethylene, which must be released in great quantities as vegetation burns.

Surprisingly, the flowers of grass trees are small, cream-colored stars arranged in spirals along the dark spear by the hundreds of thousands. A spear may stay in bloom for weeks, perfuming the mid-morning air with scents evocative of honey and musty old books. Grass trees are living reminders of how dynamic a plant can be once it is committed to reproduction.

Has science spoiled the beautiful mystery of flowering with its experiments on tissues, hormones, and entrainment? In my opinion, the ancient Greeks would have been pleased with the results of modern scientific investigations. The name of Chloris, goddess of flowers, refers to the first green shoot, and the old myths say that she was helped by Horae, the daughters of Zeus, who controlled the seasons and represented the hours in a day. As the developing meristem responds to cues of light, water, or temperature, the pagan faith in natural order is vindicated in every garden.

Copyright notice: Excerpted from pages 54-66 of The Rose's Kiss: A Natural History of Flowers by Peter Bernhardt, published by the University of Chicago Press. ©1999 by Peter Bernhardt. All rights reserved. This text may be used and shared in accordance with the fair-use provisions of U.S. copyright law, and it may be archived and redistributed in electronic form, provided that this entire notice, including copyright information, is carried and provided that the University of Chicago Press is notified and no fee is charged for access. Archiving, redistribution, or republication of this text on other terms, in any medium, requires the consent of the University of Chicago Press.

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Peter Bernhardt
The Rose's Kiss: A Natural History of Flowers
©1999, 276 pages, halftones and line drawings
Paperback $16.00 ISBN: 0-226-04440-8

For information on purchasing the book—from bookstores or here online—please go to the webpage for The Rose's Kiss.

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See also:

• Thomas Christopher, In Search of Lost Roses (read an excerpt from the Introduction)
• Stephanie Ross, What Gardens Mean
• Our catalog of books in the Biological Sciences
• Other excerpts and online features from University of Chicago Press titles