It's composite week on the blog, (see Asteraceae post from yesterday), and I'm continuing on the theme today. It's always a bit of a shocker that lettuces are in the Asteraceae along with sunflowers, zinnias, daisies and thistles. They don't even have flowers, right? Well....actually they are, and they do.
Garden lettuces all belong to the species Lactuca sativa. Just like there are many breeds of Canis domesticus (dog), there are many types of lettuce. Lettuces have the same body plan as all plants: roots, stems, leaves, flowers and fruit/seeds. The form of lettuce that we usually see is the toddler stage on the full life span of a lettuce plant. When a lettuce plant grows from a seed, it builds a small root that grows quickly and a few small leaves on a short stem. The lettuce then proceeds to grow very rapidly, building lots and lots of roots and leaves - but the stem doesn't grow! All the leaves are clumped together on a short stem low to the ground, forming a head of lettuce. This stage of plant growth is called a rosette, and many other plants also have this stage, for example, carrots and cabbage. Here are rosettes of green leaf lettuce, red leaf lettuce and romaine lettuce - all ready to be harvested:
|Green leaf lettuce (yes, that's my blue nail polish)|
|Red leaf lettuce. Notice the drip irrigation tubes.|
|Romaine lettuce being crowded by crab grass.|
Lettuce is harvested before it is allowed to finish its life cycle. I'd call it the veal of the plant world, except lettuce is treated more humanely. If lettuce were to mature, its stem would elongate and the plant would grow tall like other more normal plants. The top of the stem would develop into flowers (all ray-type flowers), and then it would produce seeds. Below are two lettuces I planted in the spring in my home garden, barely watered, ignored then didn't pick. They elongated and the first one is flowering. When rosettes elongate, it's called bolting.
|A bolting lettuce - quick, catch it!|
|Another one trying to escape|
That milky sap contains alkaloids, or molecules that taste bitter. There are more alkaloids in bolting lettuces, so they can be unpleasant to eat. Other members of the genus Lactuca contain more alkaloids, and some of these are psychoactive. They can induce a very mild euphoria and lethargy. Edible lettuce doesn't really have this quality, but because it's a close relative of the other Lactucas, it has been regarded by some cultures as sleep-inducing and has been sometimes served at the end of the night-time meal. From the plant's perspective, it's making compounds that taste bad to insects or humans, and it protects itself from being eaten. From our perspective, it's making something potentially useful. These alkaloids could be isolated and studied for medicinal purposes.
Even the heat-tolerant lettuces on the farm will fade soon. Lettuce season will be over until later in the summer and early fall when it has hopefully cooled off a bit.