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Five hundred years ago, English poet Thomas Tusser published this now-famous couplet: “Swéete April showers/Doo spring Maie flowers.” 

As home gardeners, we recognize the importance of water for plant growth. But is April rain the sole reason that flowers bloom in spring?

In the metro Richmond area, the average rainfall in the month of April is 3.2 inches. We know that water stored in the soil enters a plant’s stem through its root system and travels to its leaves where photosynthesis takes place. 

While water is critical to plant growth, other important environmental conditions include sunlight, temperature and soil quality. 

But what processes initiate growth inside a plant? From children’s rhymes to botanists, asking questions leads to a better understanding of the internal mechanisms of plant development.

Mary, Mary, quite contrary, how does your garden grow? 

In the early 20th century, biologists wanted to better understand an intriguing event in the life of a plant: its transition to flowering. 

In the 1930s, Russian biologist Mikhail Chailakhyan postulated that a substance, which he called “florigen,” initiated flowering, although neither he nor the biochemists who followed could extract this hypothesized hormone-like molecule from plant buds. 

It wasn’t until 2005 that Professor Ove Nilsson of the Umeå Plant Science Centre at the Swedish University of Agricultural Sciences proved Chailakhyan’s theory. 

According to Nilsson, “We discovered that the genes that determine when flowering occurs are active in the leaves, not in the tips of the shoots where the actual flower opens. 

“The gene that we found produces signal molecules that are conveyed from the leaves to the tips of the shoots, where they control the formation of proteins that in turn are responsible for the actual flowering.” 

This signal molecule is a type of messenger ribonucleic acid (RNA), a tiny piece of the genetic material that controls the formation of proteins. The time at which flowering occurs is preprogrammed in a plant’s genetic code similar to the way the time of sexual maturity is determined during human development. 

Hickory, dickory, dock, the mouse ran up the clock

In a paper published in the Journal of Plant Research in 2017, Keisuke Inoue and two other Japanese researchers noted that plants have an internal circadian clock, like humans do, which allows them to anticipate and prepare for daily and seasonal changes in their environments. 

The clock enables plants to increase their survival by adjusting their behavior as their environment changes — in a feedback loop. 

The genes related to photosynthesis, for example, are controlled by the circadian clock and respond to environmental changes. The circadian clock in plants also regulates the development of the plant during its life cycle, including flowering. 

Numerous different proteins work in coordination with the circadian clock and light-signaling pathways to activate transcription of the gene, which causes the plant to flower. 

Can you or I or anyone know how oats, peas, beans and barley grow? 

Photoperiodism is the physiological reaction of plants, animals and other organisms to the relative lengths of light and dark periods. The genetic feedback loops controlled by the circadian clock are highly related to these external periods of light and darkness. 

For example, peas typically blossom earlier than beans. Daffodils bloom in the spring, followed by roses in the summer and chrysanthemums in the fall. Summer grain crops include hulless oats, while colder weather brings winter wheat and winter barley. 

Long-day plants flower in spring and summer, while short-day plants flower in fall and winter. 

The plants that can bloom in any season are called day-neutral plants. For day-neutral plants, vernalization — the plant’s exposure to the necessary length of cold temperatures — induces the plant’s flowering process.