David Beeson, May 2023
While it is photosynthesis that captures the sunlight energy and converts it into chemical energy in sugars, the flowers drive reproduction and evolution. There are a few variants of photosynthesis (CAM etc – see previous articles) but huge numbers of flower design variations. Some flower stems hold a single flower and others multiples.
But what is a single flower? It is a set of, taking a simplistic view, sepals, petals, stamens and carpels. One set = one flower. With members of the dandelion family, each flower stem holds a couple of dozen small, strap-like, yellow individual flowers. A daisy has white, strap-like flowers around the edge (ray florets) and minute, yellow disc flowers (disc florets) in the centre.
Best to dissect some of the disc and ray florets and note the hair-like sepals, the petals that are joined together and spot the minute stamens and protruding stigma of the carpel. With the daisy, the ray florets are usually sterile, so may lack carpels.
A daffodil, or tulip or rose has large, colourful single flowers. The dandelion has multiple flowers on a single main flower stem.
The spot where the floral parts are attached to the stem is called the receptacle. Sometimes this can wrap around the carpel, which then lies beneath the sepals, petals and stamens. Apples and pears are examples, and a brief look will show the sepal remains with the fruit beneath. Such a situation is called an inferior ovary, because it is below the other floral parts. If it sits above the sepals etc it is a superior ovary.
The receptacle is like the human placenta in that it supplies nutrients to the flower(s).
The carpel is composed of one or more ovules (containing the female egg plus other structures), surrounded by the ovary wall with the stigma (receptive to pollen) and the style that joins the stigma to the ovule. Pollen germinates on the stigma and forms a pollen tube that generates several male gametes. The pollen is NOT a gamete but, technically, a spore.
When the pollen tube reaches the ovule one male gamete will join with the egg cell BUT another joins with other ‘female’ cells to form an endosperm. The endosperm may develop to form the white of a coconut seed or the flour of a grass seed. It may not develop and is lost in most plants.
So, flowers are complex. Holly plants are either male or female, so have single sex flowers. Some plants lack sepal / petals / stamens / carpels. Some are single others have many flowers on a receptacle. You name it, plants do it!
WWW.NWHWILDLIFE.ORG is the homepage.