It takes two flowers to make a squash

June 8, 2012

A summer squash plant with both pistillate and staminate flowers. This is a yellow squash, as you can see from the ovaries of the pistillate flowers.

Squashes, melons, pumpkins, cucumbers, and gourds all belong to the squash family, Cucurbitaceae. There is a common pattern of the flowers that children enjoy finding, and that often escapes adults. I’ve pointed it out to many long-time gardeners, who hadn’t noticed it before.

This is a young pistillate flower of a patty pan squash. The ovary is green now, but it will turn white as this squash matures.

Most members of this family are monoecious, which means each plant has flowers with only stamens along with other flowers which are only pistillate. These are commonly called male and female flowers. They are easy to tell apart if you look beneath the corolla. The ovary is inferior (located beneath the other flower parts) or, to put it another way, the other flower parts are epigenous (they sit on top the ovary).

 If you want to find the pistillate (aka female) flowers, just look for a tiny ovary – a baby squash, cucumber, etc. – on the stem under the corolla. You can find the little ovaries well before the flowers open, so it is easy to see which flowers will produce the desired fruit. The mature ovary of a flowering plant is a fruit, so to a botanist, squashes, cucumbers, and melons are all fruits.

The staminate flowers of the squash family have a plain stem beneath the corolla.

The staminate flowers have a plain stem beneath their corolla. Inside the filaments and anthers of their stamens are joined together into a knob-like structure that resembles a pistil. Inside the pistillate flower’s corolla, you can see the three-carpellate structure of the pistil. There are three stigma lobes that have two branches each. The fruit shows the three carpels as well. Look at a cross section of a squash or fruit of other family members to see this.

This is a staminate squash flower that has been split along the corolla and opened to show the fused anthers and filaments of the stamens.

The stigmas of the pistillate flower have several lobes. This flower had bloomed, and its corolla was removed to show the stigmas.

The next question that comes up is often “Why doesn’t my squash plant produce more squashes?” Sometimes the temperature is to blame. It affects the sex of squash flowers in ways that aren’t always obvious. When I lived in the mountains of Colorado, I found that although zucchini plants would grow, they seldom produced fruits. The plants would form female flowers, but seldom have staminate ones, so pollination didn’t happen. The cold soil temperatures were to blame. With other members of this family, cold temperatures cause only staminate flowers to form. You can read more about this on the website of the Ontario Ministry of Agriculture, http://www.omafra.gov.on.ca/english/crops/facts/00-031.htm

Conversely, temperatures above 95 degrees F can also cause flowers to drop instead of developing. There could be a number of factors operating in this case, including moisture stress.

Although squashes and begonias don’t commonly come to mind as relatives, if you look at the flowers of a begonia, you can see the same pattern – monoecious plants with inferior ovaries. The begonia family and the squash family both belong to the squash order, Cucurbitales.

In this view of begonia flowers, the staminate flower is on the top. It has a plain stem. The pistillate flower below has a green, winged ovary.

A front view of begonia flowers. The pistillate flower is on the left. The staminate flower has four tepals; the pistillate has five.

 

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Blooming trees that are hard to see

March 21, 2010

Happy equinox and happy tree watching!! Some trees in your area may have already bloomed, and others may be still to come. Some trees put on such a lovely show that no one can miss their blooms. For others, it takes a sharp eye and sometimes a hand lens to see the blooms. I’d like to show you the details of a few of them. For this post, I’ve chosen ash and elm trees.

Elms, genus Ulma, have their own family, Ulmaceae, which is part of the Rosales order in the rosid branch of flowering plants. They are very early bloomers, and as a result they may have their blooms frozen. If they are able to form fruits, these grow before the leaves. The trees have an early display of “spring green” fruits, which then turn brown and blow away as the new leaves are emerging.

The staminate and pistillate flowers of Siberian elms open while the weather is still freezing.

What’s the advantage in blooming so early and risking a late frost? These trees are wind-pollinated. The leaves would block the wind from the branches where the flowers form. Instead the trees take the risk and produce large numbers of fruits (“elm seeds”) as a trade-off.

The flowers of elms are tiny clusters of staminate and pistillate flowers that have tiny tepals, not showy petals. The stamens have dark anthers and the pistillate flower has two tiny, furry stigmas, usually light-colored. The ovaries start to enlarge soon after the trees bloom, and you can see the enlarging green disk that will become the fruits.

The samaras of elms grow quickly and mature before the leaves open. This is a Siberian elm.

The fruit is known botanically as a samara, which literally means “elm seed” in Latin. A samara is a winged fruit that is wind-dispersed. Elm fruits have a single seed surrounded by a membranous ring.

The flower clusters of the American elm are more open. Each flower has a long stalk.

 

The developing ovaries of the American elm are covered with short hairs.

Ash trees, genus Fraxinus, belong to the olive family, Oleaceae, which is part of the Lamiales order in the asterid branch of flowering plants. They bloom later than elms, but still quite early. Ash trees are dioecious, which means that they have their staminate and pistillate flowers on separate trees. The staminate or male trees are often planted as street trees. They have the advantage of not producing fruits, so not requiring a lot of clean-up. Their distinct disadvantage is that they produce abundant allergy-triggering pollen.

The staminate inflorescences bear two stamens per flower, like all the olive family. They start as tight, globular clusters. The more open inflorescences of pistillate flowers show the tiny green ovaries that rapidly enlarge. The fruits are another samara.

Early on, the immature stamens form tight clusters as they emerge from their buds.

Later the stamens mature as the cluster opens. The pollen flies as the anthers split open.

This pistillate ash tree has the old, dried fruits (samaras) from the previous year, along with the green inflorescence of this spring.

  


Seen any good grass flowers lately?

July 10, 2009

Our summer is rushing along, as summers usually do. With the abundant rains this year, grasses are growing profusely and the grass flowers have been a treat to see. If you ask most people, they will say that grasses don’t have flowers. It all depends on what you consider a flower. The common notion is that a flower has to be colorful and showy. That’s fine if the flower is pollinated by an insect or other animal, but wind-pollinated flowers don’t bother with all that extravagant use of resources. Their flowers are the most basic models – tiny petals or none at all, no scents, no nectar. The wind doesn’t work any better with those things than it does without. All it takes to be a flower is a stamen or a carpel, and grass flowers have both – one to three stamens and a two-carpellate pistil usually.

Here are typical grass flowers. The anthers are yellow and the stigmas are feathery and white.

Here are typical grass flowers. The anthers are yellow and the stigmas are feathery and white.

I’ve been asked what grass flowers look like, and that’s a good question. Without a hand lens or other magnifier, it is hard to see them at all. Basically grass flowers form within a series of bracts – small modified leaves, which are usually green. This little package of flowers and bracts is called a spikelet. Each spikelet can have one to several flowers + bracts stacked together. When the flower is mature, a pair of little scales (the lodicule) at the base of the ovary swells and prys the stack of bracts apart. The stamens, typically three of them, dangle out on long, flexible filaments. The anthers are large compared to the size of the whole flower. They have to be to shed enough pollen. Wind tends to scatter pollen and dilute it. The stamens are the easiest part of the grass flower to see. The pistil typically has two styles and two feathery stigmas. If you would like more details on grass flowers see http://www.backyardnature.net/fl_grass.htm

The feathery stigmas have a large surface area to snag pollen. Their structure may also alter air flow, making it more turbulent and promoting pollen contact with the sticky stigmas. The stigmas are often white, but there are many colors in various grass species. After the grass flower has bloomed, the bracts close back up and there is little of the flower to see on the outside of the spikelet. Sometimes the stamens remain for a short while after the bracts close. Inside, the ovary of the flower is developing into a closed, dry fruit. The layers of the ovary wall adhere closely to the seed, so the whole thing is commonly called a seed or a grain. A kernal of wheat, for example, is technically a grass fruit.
This switchgrass has orange anthers and pink stigmas - pretty fancy for a grass.

This switchgrass has orange anthers and pink stigmas - pretty fancy for a grass.

The grass family is one of the largest of the flowering plant families, so my photos show only a tiny fraction of the variety of grass flowers. It’s another good challenge for field work – find the grass flowers. Happy hunting!

Many flowers in this grass inflorescence are blooming. 

Many flowers in this grass inflorescence are blooming.