A Spanish Broom with its many straight green stems. Photo by Robert Dryja/ladailypost.com
By ROBERT DRYJA
Los Alamos
An exploratory walk can lead to plants that look very different from one another or very much alike. The Spanish broom is an example. It is a bush that may grow six to eight feet high. It is somewhat circular in shape and can have hundreds of straight slim stems.
The stems are up to an eighth of inch wide, green colored, and two to three feet long. A stem may have few small leaves growing toward it base, (see Picture 1). It is at its flowering peak in June.
The perennial sweet pea also is at its flowering peak in June. This is a much smaller plant compared to the Spanish broom. It is a vine that may lie on the ground or may grow upright if it can attach itself to another plant growing upward. It has a single stem that is relatively flat in shape rather than circular. The stem has pairs of large leaves appearing every few inches. It may grow to be two or three feet in length, (see Picture 2).
A Spanish broom may be covered with hundreds of bright yellow flowers. A sweet pea vine also may have many purple or white flowers growing in clusters, (see Picture 3).
Given the different shapes and sizes of the stems and color of the flowers, the impression is that Spanish Broom and sweet peas are distinctly different from one another as plant species. However, two aspects suggest they are not totally different.
First, the flowers have a similar structure. A flower is composed of five specialized petals. There is one large upper petal. It is called a “banner” for the Spanish broom and a “standard” for the sweet pea. There are two smaller side petals called “wings” for both kinds of flowers. Finally, there are two narrow petals growing against one another and together are called a “keel”. The keel encloses parts called the stamen and stigma. The stamen produces pollen while stigma produces seeds, (see picture 4).
Second, both produce seed pods that look the same, (see picture 5). The shell of a pod has the same shape for both kinds of plants and may contain 5 to 10 seeds. A pod grows from a keel and may reach about four inches in length. A pod initially is green and becomes brown when it matures and dries. The seeds are released when the sides of a dry pod split apart. A Spanish broom bush may produce 7,000 to 10,000 seeds, given the huge number of flowers that grow on it. A sweet pea plant does not have as many flowers and so does not produce as many seed pods. An estimate suggests that up to 300 seeds may be produced in thirty pods of a sweet pea plant.
A Spanish Broom and Sweet Pea therefore look like similar species because their flowers and seed pods look similar. However, the different size and shape of their stems and their leaves make them look very different. How many ways do plants need to look similar to be considered a closely related species?
A sweet pea vine has broad leaves growing in pairs along its flat stem. Photo by Robert Dryja/ladailypost.com
The large ‘standard” petal of the sweet pea flower stands out while its two wing petals are small. Photo by Robert Dryja/ladailypost.com
A Spanish broom flower has one large “banner”, two smaller wings and a pointed keel where seeds will develop. Photo by Robert Dryja/ladailypost.com
Maturing pods are growing adjacent to the large leaves of the sweet pea plant. Spanish broom seed pods look very similar. Photo by Robert Dryja/ladailypost.com