Summer squash season has come to an end in the S&W garden. Removing these gigantic zucchini plants gave me a chance to investigate their structure, in particular their stems and leaves. Some of the stems on this plant were thicker than a garden hose, and they even dripped out water as I was chopping them up for the compost!
So, what’s going on in these plant “pipes”? And how do they work?
I don’t remember much about plant biology from my science classes. Human biology is more my area of expertise. One thing I do recall learning about plants (maybe from one of my high school science teachers) is the phrase: “xylem up, and phloem down”. Beyond that, I needed to do some reviewing.
It turns out that the outer layer of the plant’s stem goes by the same name as the outer layer of skin in humans: it’s the epidermis. This doesn’t seem too difficult so far…
Plants and humans have another anatomical feature in common: they both have circulatory systems to move things around between their different parts. To take a look at the circulatory system of a plant, it helps to refer to an example, like this cross-section view of the zucchini squash stem. This first picture is taken near the base of the stem (closest to the root):
In this photo, you can see how the squash stem is organized into more-or-less triangular or teardrop-shaped segments. Each of these segments contains what is known as a vascular bundle: a collection of larger tubes (called xylem) that bring water and minerals from the plant’s roots to its leaves, and another bunch of smaller tubes (called phloem) whose job is to bring sugar from the leaves (where photosynthesis happens) to the rest of the plant to allow it to grow. You really need a microscope to see all these structures clearly, or simply look online to find images that have been taken through microscopes.
This second photo is a cross-section cut further along the zucchini squash stem, closer to the leaf:
Here, the xylem and phloem are arranged a bit differently around the outer rim, but you can still get the idea that there are small tubes or channels for things to flow “up” the plant towards the leaves (the xylem), and to flow “down” the plant to distribute sugar (the phloem) to all the plant parts.
A fun, kid-friendly way to demonstrate how water moves up the xylem of a plant (through a process called capillary action) is to try this experiment with stalks of celery placed in water containing drops of food coloring.
I noticed a couple of things about zucchini squash plant leaves, too. First, they are enormous, almost big enough to be useful as umbrellas. They are beneficial for shading the soil and any fruit that is developing on the plant. Zucchini leaves need lots of sunshine, but the zucchini fruit is like us: it prefers not to get sunburned! Shade provided by the leaves prevents weeds from growing, and it also helps the soil retain moisture.
The second thing I noticed is that squash leaves (and stems) are prickly. Close contact with these prickles can cause a pretty nasty rash on your epidermis (I just had to throw that term in again!).
If you peel away the prickles (or even if you don’t), squash stems and leaves are apparently edible, but I don’t think I’m quite brave enough to try any of these recipes.