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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A Rare Plant Treat

July 6, 2010

I was in Seattle recently and enjoyed a visit to the greenhouses at the University of Washington. I was delighted to find a collection of Welwitschia plants there, one of which was forming cones. This bizarre plant of the Namibian Desert belongs to the gnetophyte lineage of seed plants. It is one of three wildly different branches of this clade.

Welwitschia plants in the UW greenhouse have tall pots to accommodate their long tap roots.

The gnetophytes are one of five extant lineages of seed plants. The others are the cycads, the ginkgo, conifers, and the angiosperms, aka flowering plants. The angiosperms greatly outnumber the others, but that has become the case in the Cenozoic Era. In the Mesozoic Era, there were many other types of gnetophytes, but presently the three lineages are the genus Ephedra, genus Gnetum, and genus Welwitschia.

Although there are about 60 species of Ephedra and about 30 of Gnetum, there is only one species of Welwitschia, W. mirabilis. Ephedra species are native to arid environments in North and South America, Africa, and Eurasia. They have greatly reduced leaves and look like a bush of tough, narrow stems. Gnetum is the odd one on environment. It grows in the tropics of Indonesia, the Philippines, and parts of Africa. Most of its members are vines, although a few are trees or shrubs. Their leaves look very much like those of angiosperms.

Ephedra, a relative of Welwitschia, growing in Utah. The leaves of this bush are reduced to tiny scales.

Gnetophytes were once thought to be closely related to flowering plants, but the DNA told a different story. They are now considered to be more closely related to the conifers. Characteristics that all gnetophytes share include opposite leaves and having staminate and ovulate reproductive structures on separate plants, i.e. they are dioecious.

Welwitschia grows only two leaves, not counting its two seed leaves, in its whole life time. The leaves arise from a woody stem that has a sort of upside down cone shape. The plants are estimated to live about 1000 years, judging from their growth rates and the length of the leaves on wild plants. The leaves grow from their bases and often split so that it isn’t easy to see that there are only two.

The stem of Welwitschia is woody and has a roughly inverse cone-shape.

The two leaves of this Welwitschia plant are split into several strips. The top of its woody stem is visible.

One of the plants in the UW greenhouse had formed cones. It is a male plant, and there are stamens showing near the bottom bracts. The branch that bears the cones grows from the top of the woody stem, near the base of the leaves. I had read about their cones, but never seen them “in person” before, so finding them was a real treat. 

These are staminate cones of Welwitschia. Their stalk grows from the top of the woody stem.


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.


(Disappearing) phyla of the plant kingdom

March 20, 2009
whiskfern

Whisk ferns are a part of the fern lineage. They are no longer a phylum.

I noticed last year as I was writing my book, Kingdoms of Life Connected, that plant systematics textbooks were not using phyla, except for the bryophytes in some cases. What’s going on here? It is nothing less than a new view of the plant kingdom and of the classification of life.

Our traditional view was set by Linnaeus, back in the mid 1700’s. His system was based on the assumption that all life had been created simultaneously and that it was unchanging. He based his work on the appearance of the organisms and placed them in the hierarchical categories that we still (sometimes) use – kingdom, phylum, class, order, family, genus, and species.

Now botanists look at the lineages for each group of plants. For example ferns can be seen as members of the plant kingdom (embryophytes), vascular plants (tracheophytes), true-leaf plants (euphyllophytes), the fern lineage (monilophytes), and finally the branch of the leptosporangiate ferns, sometimes called the true ferns. This is a much richer view than a simple box labeled “ferns.”

What Linnaeus missed is the history of each organism. Darwin bought forward the important idea that organisms have histories. Some share a recent common ancestor, others do not. The history of an organism doesn’t show up in a row of boxes. Instead it must be displayed as a branching diagram that shows which organisms are more closely related. 

Getting back to the plant kingdom and how we introduce it in the elementary classroom – usually the plant kingdom was broken into phyla (or divisions if you prefer the traditional name). Most of children’s literature – the small body of learning resources that actually address the plant kingdom – use phyla, whereas college texts and professional botany writings have largely discarded that category for plants.

Why have most of the phyla names been dropped? They didn’t work with the new view. The horsetails were previously placed in their own phylum, but they are embedded in the monilophytes, the fern lineage. So are whisk ferns, the psilophytes. The phylum name is even less useful for whisk ferns because it excludes their close relatives, the grape and adder’s tongue ferns.

Will phyla come back? It’s not impossible, but it may take a while for botanists to settle on what to call a phylum. Is it a branch of the ferns, the whole fern lineage, or the euphyllophytes? If children know the main branches of plant life, they are well-prepared whatever system develops. The animal kingdom, by the way, has kept its phyla, although they are now grouped into different lineages than they were a decade or so back.

Another term to discard is “seedless vascular plants.” The vascular plants have two branches, the lycophytes (club mosses and their relatives) and the euphyllophytes. The latter has two branches, the monilophytes (ferns in the broad sense) and seed plants. Club mosses shouldn’t be in the same category as the ferns, and “fern allies” mixes unrelated lineages.

For children to see the current view of the plant kingdom, they need a branching diagram that shows who is related to whom. Rows of boxes are out, phylogenies are in. A phylogeny is a branching diagram that illustrates a hypothesis about the evolution of organisms. Actually, the word applies both to the hypothesis and to its illustration. For more about current phylogenies, see my book, Kingdoms of Life Connected: A Teacher’s Guide to the Tree of Life and take a look at the charts that are available for free download from my website, http://www.bigpicturescience.biz.

 

Another term for a phylogeny is a Tree of Life or, informally, a family tree. The Tree of Life web project (http://tolweb.org/tree) has a great illustration in its home page. If you go to the page for the plant kingdom (aka embryophytes), you will see the extant lineages and a number of extinct ones. See http://tolweb.org/Embryophytes/20582. It is always thought-provoking to see the extant lineages in the matrix of extinct ones.


What should you do with your old plant kingdom charts? Keep them for historical perspective. Children’s literature still shows older classifications. Older charts also help children see the changing nature of science thought. Just make sure that children have working knowledge of the new, phylogenetic view of life.