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Botanist Marie Clark Taylor and the Impact of Light on Plant Processes: Home

Cosmos bipinnatus; photo courtesy of Flickr cc/Stefan Rohner
Cosmos bipinnatus is one of the plants used by Dr. Taylor in her research; photo courtesy of Flickr cc/Stefan Rohner

Dr. Marie Clark Taylor (1911–90) was a passionate educator and botanist. The first Black woman to receive a PhD in botany in the United States and the first Black woman to receive a PhD in science from Fordham University (1941), Dr. Taylor had a decades long career teaching undergraduates at Howard University, where she became the first Botany Department chair. Among numerous contributions to education, Dr. Taylor promoted the use of plants as accessible materials to teach about cellular life processes and the value of hands on learning.

While many of Dr. Taylor's extraordinary accomplishments centered around her life as an educator of scientists, her own scientific research is also notable.  Her doctoral research and dissertation explored photomorphogenesis, the effect of light on the structure and life processes of a plant, outside of the more familiar process of photosynthesis.

Photomorphogenesis and Horticulture

Light is a means by which plants perceive their environment. Understanding this relationship of light to plant development is what has allowed gardeners to select the plants that will prosper in their particular garden conditions. It also underpins the very idea of keeping plants in a conservatory or home and creating the best possible growing environment in an artificially created habitat.

There are many types of photomorphogenic responses in plants. Dr. Taylor examined the effect that light photoperiods (periods of daily light) have on the development of the cells that give rise to flowers. Studies of this type that explore and explain a plant's relationship to the fundamentals of its environment are the foundation of horticulture.

The vivid red flowers of Salvia splendens; photo courtesy of Flickr cc/ James St. John
The vivid red flowers of Salvia splendens; photo courtesy of Flickr cc/ James St. John

The plants used by Dr. Marie Clark Taylor in her research

Dr. Taylor's doctoral research studied the plants scarlet sage (Salvia splendens), cosmos (Cosmos bipinnatus) and orange cosmos (Cosmos sulphureus). She subjected the plants to different lengths of regulated light (6, 10 and 16 hours) and measured the response in seed and flower production. The sage plants produced the most flowers under the 10 hour photoperiod and the least flowers at 16 hours. The cosmos plants continued to increase flower production with added daylight but the flowers declined in quality at the longest photoperiod.

These are plants that you can grow as garden annuals in the New York City area and observe for yourself! Annual plants will only live for one growing season but can add colorful and profuse flowers to your garden. They may be started from seeds indoors in spring and then grown in a garden bed or container outdoors during the summer. Dr. Taylor demonstrated that flowering and quality may be reduced in these plants by extended periods of sunlight (long photoperiods). Starting the seeds indoors in the spring will allow you to enjoy flowers in the garden earlier, before the longer day length affects them in mid-summer.

Scarlet sage (Salvia splendens)

Start this warm weather, annual plant from seed indoors 6 to 8 weeks before the last frost date in your area. (In the New York City area, start your seeds around mid- March.)  Lay the seeds on top of moist seed-starting medium and do not cover them; they need light to germinate. Keep the seeds at a temperature of around 70ºF. and germination should take place in about two weeks. Harden off the seedlings and plant your scarlet sage out in a sunny to partly sunny position, in well drained soil, once the risk of frost has passed. It is not a tricky plant to grow but appreciates moisture. It will grow up to two feet tall, with small, tubular, bright red flowers on tall stalks in June through the fall. There are cultivars in various colors and shorter growing habits that are ideal for container growing.

Cosmos (Cosmos bipinnatus) and orange cosmos (C. sulphureus)

Start these warm weather, annual plants from seed indoors 6 to 8 weeks before the last frost date in your area. (In the New York City area, start your seeds around mid- March.)  Sow seeds, barely covered, in moist seed starting medium and keep at a temperature of around 70ºF.  Keep moist and seeds should germinate in a week  to 10 days. Harden off the seedlings before planting outdoors after the last frost date.  Choose a full sun position in average soil. You will have daisy-like flowers with yellow centers in shades of pink and white (or orange in the case of C. sulphureus) beginning in June on stems from 2 to 4 feet in height. Deadhead for continued bloom until the fall or cut back plants once the early flowering season has passed and seeds have formed, to 18 inches, for regrowth later in the season. Self seeding may generate more plants than you want in a garden area.

You will find additional help with getting your seeds off to a great start in our guide Starting Seeds Indoors. If you prefer to grow houseplants, see the Explore the Impact of Light on Houseplants Tab at the top of the page to learn more about how Dr. Taylor's work added to our understanding of houseplant care.

Cosmos sulphureus; photo courtesy of Flickr cc/ M's Photography
Cosmos sulphureus; photo courtesy of Flickr cc/ M's Photography


Alexander, Christine. “Marie Clark Taylor and The Wonder of ‘Aliveness!".” Fine Gardening, Fine Gardening, 8 July 2021,

Dupree, Gus. “Remembering the Legacy of Marie Clark Taylor, Ph.d., a Trailblazing Female Botanist of Color.” The Observer, The Fordham Observer, 8 Dec. 2021,

McCleary-Harris, Sierra. “Remembering the First Woman to Earn a Ph.d. in the Sciences.” Fordham Newsroom, Forham Magazine, 30 Mar. 2021,

Pizza, Riley. “Marie Clark Taylor: Exposing Plants to Different Day Lengths Changes Their Flower Development.” Project Biodiversify, Project Biodiversify, 24 Nov. 2020,

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