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.

 


Botany marches on – Part 1, basal angiosperms and monocots

April 13, 2010

Warning! I’m going to get into some detailed botany here, so if you are not deeply into the flowering plants and their classification and you do not own my book, A Tour of the Flowering Plants, you may not wish to wade through all this. It is basically for advanced elementary and secondary Montessori botany studies and for anyone else that owns the book. A Tour of the Flowering Plants is still quite useful to show the appearance of angiosperm families, whether or not you are concerned about the latest classification.

Last fall, the Angiosperm Phylogeny Group published a third report concerning the classification of the orders and families of flowering plants. The report, published in the Botanical Journal of the Linnean Society, is called APG III. There is a summary of APG III on Wikipedia. http://en.wikipedia.org/wiki/APG_III_system My book, A Tour of the Flowering Plants, is based on the 2003 report from this group, which is called APG II. The book has some further advances that were published on the Angiosperm Phylogeny Website of Peter Stevens. http://www.mobot.org/MOBOT/Research/APweb/welcome.html

I’ll go over the APG III changes, beginning with this post. If you own my book, you can decide if you want to add notes to it. You may contact me via my website (www.bigpicturescience.biz) if you would like a list of the pages and changes for bringing A Tour of the Flowering Plants in line with APG III.

Will the changes keep coming over the years as we acquire more and more DNA data? I think that future changes will be modest and will not affect the general structure of the angiosperm tree. There aren’t that many unplaced groups left. There will likely be little surprises, like one I give below for Nymphaeales.   

For now, let’s start with the first branches of the angiosperms, grouped as the basal angiosperms in A Tour of the Flowering Plants. The Nymphaeales got another family, Hydatellaceae, which was formerly placed in the grasses. This shows how much DNA studies can reveal, and what studies of morphology may not be able to distinguish. This family is tiny, both in size and numbers of species. The only reason I mention it is the idea that when plants adapt to living in water (or any other extreme environment), they often change form so much that they don’t resemble even their closer relatives. You can see the plant here: http://www.ubcbotanicalgarden.org/potd/2007/03/hydatellaceae_1.php

The order Chloranthales was previously unplaced – no one was sure what its closest relatives are. Now it is considered to be a sister group to the magnoliids. The magnoliids themselves have not changed in APG III. This branch of the flowering plants includes the laurel and black pepper families, as well as the magnolias. Although their seeds have two cotyledons, they are not closely related to the eudicots. The eudicots are the traditional dicots minus the magnoliids and the basal angiosperm lineages.

In A Tour of the Flowering Plants, I used terms for branches of monocots that have since disappeared. You don’t have to worry about whether to call the Liliales and Asparagales “lilioid monocots” or “petaloid monocots.” Just call them monocots and go on. The only lineage of monocots that gets a special name now is the commelinids. “Lilioid” and “petaloid” should be understood as informal terms that refer to plants that were traditionally lumped in the lily family. Most of them have large, showy tepals.

The major change in the monocot is the grouping of several small families as subfamilies under the enlarged families Amaryllidaceae, Asparagaceae, and Xanthorrhoeaceae. This is all within the order Asparagales. Here’s the breakdown:

The enlarged Amaryllidaceae has the agapanthus subfamily (Agapanthoideae, equivalent to the former Agapanthaceae), the onion subfamily (Alliodeae, equivalent to the former Alliaceae), as well as the amaryllis subfamily (Amaryllidoideae, equivalent to the former Amaryllidaceae). The members of this enlarged family have their flowers in umbels that are enclosed by two bracts when the flowers are in bud.

The enlarged Asparagaceae is really big. It has the Brodiaea subfamily (Brodiaeoideae), the scilla subfamily (Scilloideae, which includes the former hyacinth family, as the tribe Hyacintheae), the agave subfamily (Agavoideae, which includes the former Agavaceae), the Nolina subfamily (Nolinoideae, equivalent to the former Ruscaceae), as well as the asparagus subfamily (Asparagoideae). Yet another subfamily holds several Australasian species such as the cabbage tree, Cordyline.  

Asparagaceae members have flowers in racemes or in umbels that have three or more bracts at their base. The umbels, if present, do not have the pair of enclosing bracts seen in Amaryllidaceae.

The enlarged Xanthorrhoeaceae (the grass tree family) includes the daylily subfamily (Hemerocallidoideae) and the asphodel subfamily (Asphodeloideae), as well as the grass-tree subfamily. I did not include the grass tree family in A Tour of the Flowering Plants because it is native to Australia and not commonly used in North American landscaping.

The cattail family, Typhaceae, got a second genus, Sparganium, the bur-reeds. It’s not hard to see these two aquatics as relatives.

I’ll address the APG III changes to the eudicots in another post.


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.

  


Poinsettia flower hide-and-seek

December 4, 2009

Poinsettia plants (Euphorbia pulcherrima) have become a traditional winter holiday decoration. If you have one available, try to find its flowers. It isn’t an easy task. While every species of flower is a variation on the basic theme of flower parts, the poinsettia and other members of the euphorbia family are particularly unusual.

First, I should define what I mean by a flower. Botanically it is the reproductive structure of a flowering plant. A “complete” flower – one that has all the usual parts – has a calyx made of sepals, a corolla made of petals, stamens, and a pistil. Not all these parts are necessary for the flower to function – that is, to make a fruit and seeds. To be a flower, the minimum requirement is at least one stamen or one simple pistil (carpel). All those other structures just enhance the function of the stamen or the pistil. The stamen produces pollen, which holds the sperm cells that will fertilize the egg cell and endosperm in the ovule. If the ovules are fertilized, the ovary of the pistil matures into a fruit with seeds inside.

If you want to figure out the poinsettia’s flowers, it really helps to see the developmental sequence. Poinsettias are usually sold when the flowers are fully formed, so I’ve provided some photos of earlier stages to help you see what’s what.

In mid-October, these poinsettia plants are all green. They haven't started forming bracts.

First, let’s take a look at the parts are NOT flowers. Hint: look for the most conspicuous and showy parts. The colorful petal-like parts are simply modified leaves that help attract pollinators. They are called bracts, and they develop when the days are short in the fall. The photo

Only ten days later, red bracts are starting to form.

sequence of poinsettias in a greenhouse shows that the bracts are leaves that are colored. Bracts have the same vein pattern as leaves and if you look at the stem, you will see that they have an alternate arrangement – one bract at each node – instead of the whorled arrangement that petals usually have. 

The bracts have an alternate arrangement on the stem - one per node.

 

Look at the photo below that shows a cluster of small green rounded structures in the center of the bracts. Are they the flower buds? No, they are developing inflorescences. An inflorescence is a group of flowers on a single stem. The botanical term for this inflorescence is a cyathium, which comes from the Greek term for a cup. As these structures mature, they become cup-shaped. The rim of the cup takes on a color, yellow to red, depending on the variety. One or two structures that look like yellow or orange lips grow on the top of the cup. While most nectar glands are in a single flower, these reward pollinators for visiting the whole inflorescence.

The little green structures in the center are the early stages of the inflorescences, the cyathia.

Each cyathium holds either stamens, a pistil, or a mixture of them both. Normally when you see stamens and a pistil, you are looking at parts of one flower. Here each individual stamen or pistil is a separate flower with its own separate stem. You can see this most clearly on the pistils. Their stems elongate and push them out of the cyathium as they mature. Many of the poinsettias sold commercially seem to have only staminate flowers. You can see the dusty yellow pollen on the anthers at the ends of the filaments. The pistils are three-carpellate – composed of three fused carpels (aka simple pistils). The style of each carpel curls open and splits in two as it matures. When the pistil first forms, you may see only the styles and stigmas. As it grows, you can see the whole ovary and its stem hanging out of the cyathium.

These cyathia have only stamens. Note the yellow anthers and red filaments. The yellow nectar glands are near the top of the cups.

In this variety, the styles and stigmas are red. The stamens are yellow. The pistils have three styles that each split in two.

Even if your poinsettia has pistillate flowers and its ovary grows, don’t expect seeds. Most hybrids with those large colorful bracts are no longer fertile. They are propagated from cuttings. Poinsettia growers seldom grow plants from seeds. If they did, they would get a range of bract colors and maturity times, rather than all the plants looking the same. In fact, poinsettia seeds are hard to find. I found only the wild type in a recent search.

The ovaries of the pistillate flowers have enlarged. Their stems have pushed them out of the cyathia.

After your poinsettia has bloomed, the bracts remain even as the cyathia drop off. You can keep it and allow it to grow. In the spring you should cut back the bracts to keep the stems from becoming long and scraggly. This allows new leaves to form. If you want the plant to form bracts and flowers again, it will have to be kept in the dark for about 15 hours a day until the bracts are colored. For more information, see http://aggie-horticulture.tamu.edu/extension/newsletters/hortupdate/novdec00/art1nov.html  

Poinsettias are rumored to be poisonous. They certainly should NOT be considered edible, but their milky sap is only mildly irritating. Finally, my thanks go to TaTonka Farms of Conifer, Colorado for allowing me to photograph their beautiful poinsettias throughout the growing season.  Happy holidays!

This variety has variegated bracts.