Where Dandelions Bloom First

For several years, I’ve noticed that in many different locations, dandelions located on a southern slope are the first to bloom in the springtime. It happens because the slope receives more direct rays from the sun, causing the soil to heat up faster. This presumably signals the plants that it’s time to grow and reproduce. It’s related to the reason why our noses, if they are sloped, get sunburnt before the rest of our faces. For centuries, people have been aware of the southern slope advantage, given that it’s been used as an optimal location for vineyards.

The dandelions on the left are growing on a ~30 degree slope. They bloomed first and are now at the seed-producing stage. On the right hand side are nearby dandelions growing on a flatter area. They are still in bloom, having blossomed after their sloped counterparts.

It’s not the kind of thing I could have noticed in my youth, given that municipalities would apply weed-killer to get rid of dandelions. Nowadays, to the dismay of herbicide manufacturers, many cities have bylaws prohibiting the use of such compounds, so dandelions abound. There are also less people who are obsessive about upkeeping weedless lawns, so in May especially, dandelions abound, and many children’s favorite color is in full display for a few weeks before they all turn white.

The seeds that are attached to the white fluffy part that helps them spread like wildfire are actually the result of parthenogenesis or virgin birth. That implies that all the seeds that a plant produces are clones —genetic copies of one another. Just last year biologists isolated the parthenogenesis (PAR) gene from the asexual dandelion that triggers embryo development in unfertilized egg cells. When the gene was deleted from dandelions by the CRISPR technique, they still produced the white sphere but one without seeds, as shown below in the picture to the right:

Image from https://www.nature.com/articles/s41588-021-00984-y

In my front yard, I enjoy having a variety of plants among my Kentucky bluegrass, but the dandelion is not my favorite. It grows at the expense of other plants that I prefer such as white clover, black medic, yellow clover, poppies and oregano. Since dandelions won’t naturally delete the PAR gene that allows them to multiply so efficiently, the key is to remove the plants before they flower. Even that can be a challenge. If you are like me and are aversive to using herbicides, then you have to pull them , making sure to pull out the entire root. Just like the lawn mower has acted as a selective force, which has shifted the dandelion population towards blooms with shorter stems, pulling weeds by hand has also had an impact. In my yard, it’s gradually given a selective advantage to plants that produce very long and deep-reaching roots. Those are more likely to leave a surviving fragment allowing the more common form of vegetative reproduction to keep propagating the weed

If dandelions don’t reproduce sexually, how do they create the variety that natural selection can then act upon? When pulling out dandelions, one observes a spectrum of root thickness among plants that are side by side in identical soil, reducing the likelihood of an environmental cause. When botanists checked for mutations among dandelions, they found a rate that ranged from 7 to 15 per 1000 individuals. A mutation is a change in the DNA code caused by either nature or by man-made products. The study from the same link concludes that the mutation rate in dandelions is directly proportional to concentrations of chromium, iron, manganese, and nickel in leaf-tissue. Another study reveals that some metals themselves cause dandelions to evolve. They don’t exclude the pollutants, but sequester them and then tolerate their presence, again in direct proportion to the amount of pollution they are exposed to.


Blossoms of Lettuce

While lettuce grows in our garden at its frantic pace, I tend to pick leaves from the core to prevent it from flowering. If not, the leaves don’t grow to size and don’t acquire the same soft texture. But after several cycles, it exhausts itself, and no matter how I pick them, the plant will grow flowering stalks and bloom.

As it bloomed this morning, I was reminded that Lactuca sativa is a member of the daisy family, Compositae aka Asteraceae. Its flowers are like a smaller, yellow version of a chicory’s (Cichoria intybus) reproductive structure. Both are an infloresecence of many tiny flowers known as florets. They each have several 5-toothed petals and the florets consist of five fused anthers that form a tube surrounding the style(female structure that connects the stigma and the ovary).

chicory inflorescence

lettuce inflorescence

Lactuca virosa (wild lettuce)

Although Carl Linnaeus originally thought it was feasible, plants cannot be classified on the basis of flower structure alone. Other structural features are used and in recent decades, botanists have also relied on biochemical and genetic comparisons.

Modified from Sneath PHA, Sokal RO (1973) Numerical taxonomy. Freeman, San

Based on mitochondrial variations, the placement of chicory (Cichorium) and lettuce (Lactuca) in different genera is justified. But chicory and lettuce, as shown from the diagram, are more closely related to each other than to dandelions (Taraxacum) or sow thistles (Sonchus) or Cupid’s dart (Catananche). Lettuce, however, is more closely related to sow thistles (Cicerbita) than to chicory.

Speaking of chicory, in the spring of my youth, my aunts, mother, neighbors and grandmother would comb empty suburban fields for young chicory (and dandelion) leaves and them use them in salads. It’s probably not a coincidence, given the ethnic origin of my relatives and neighbors, that lettuce originated in the Mediterranean area where it too was picked and eaten as a weed. Eventually, ancient Greeks and Romans cultivated lettuce, but were preceded by Egyptians who depicted lettuce on tomb paintings. However, from looking at their artworks, I have a hard time identifying it as lettuce. More certain is that much later, by 1885, the New York Agricultural Experiment Station was growing 83 distinct varieties of lettuce.

Most commercial lettuce on the North American continent is grown in California and Arizona. In the former location there have been some serious E. coli outbreaks in 2018 and 2020.  Fields where the plants were grown were close to large feedlots, which the FDA reported could have contaminated romaine lettuce with E. Coli from both dust and contaminated irrigation water from open trenches. Subsequently, there has been closer monitoring by government officials and researchers teams.

What is promising is that lettuce is being increasingly grown on rooftops and in city or home gardens, which reduces the carbon footprint from transporting lettuce thousands of miles from the western parts of the continent to the densely populated eastern parts. And it’s a lot more fun to pick it from one’s backyard than to go buy it at the grocery store.

An Isolated Plant Family

There is a family within the angiosperms (flowering plants) that, from an evolutionary viewpoint, is relatively isolated. The only family that’s somewhat closely related to it is that of the morning glory, Convolvulacea. The family in question has about 1700 species, which is not much compared to the diversity within the pea, orchid and sunflower families, which all together account for over 60 000 species or roughly 20% of all flowering plants.

Its flowers are very often star-shaped with 5 sepals, 5 petals and 5 stamens, and its stems and leaves host a variety of both toxic and useful alkaloids. If you haven’t guessed it yet we are referring to the Solanaceae family, whose most familiar members include tomatoes, eggplants, tobacco, potatoes, petunias, jimson weed and deadly nightshade (belladonna). Not many people realize that the tomato, has a close relative known as the tamarillo or tree tomato, also a native of the Andes. This shrub produces fruits of various colors, with the orange one being sweeter than the red. Currently, the tomato and tamarillo are classified in different genera, but that grouping is probably mistaken according to DNA studies.

On the left are representatives of a couple of tomato varieties from my garden. On the right is a tree tomato, which like its relative is also originally from the Andes.

What kind of alkaloids are found in Solanaceae plants? Potatoes and tomatoes both contain solanine, which has fungicidal and insect-repellent properties. Tomatine in tomatoes serves the same purpose and is also present in unripe tomatoes. If you touch the stems or leaves of tomatoes while picking them, you will cause their tiny hairs (trichomes) to release alkaloids along with oils, which give plants their distinctive scent. Unfortunately, some people are very sensitive to some of the compounds made by the trichomes, and they develop eczema: itchy skin with small, fluid-filled bumps leading to scaly skin from the release of histamines by the immune system.

Other Solanaceae alkaloids go far back in history and have proven to be beneficial. Over 3000 years ago, Egyptians added extracts of a Solanaceae family-member known as mandrake to their beer. Mandrake contains atropine, scopolamine and hyoscyamine, which block the action of the neurotransmitter acetylcholine. The trio of alkaloids, which are also found in jimson weed, belladonna and henbane, can cause hallucinations, but individually, at the right dose and for the right condition, each can serve as a useful drug. Their similar structures can be seen below. Atropine and hyoscyamine (left and top right) are mirror images of one another but not identical in the way that a right handed glove is not the same as a left handed one. Scopolamine differs in having an epoxide ring (triangular arrangement of two carbons and an oxygen) adjacent to the nitrogen ring.

Atropine can be used to dilate eyes, to treat a specific type of heart block and to treat certain cases of organophosphate poisoning. Scopolamine, in the form of a transdermal patch, is used to prevent nausea and vomiting after anesthesia or from narcotic pain medicines, and it’s also good for motion sickness. Hyoscyamine works by decreasing acid-production in the stomach, slowing down the natural movements of the gut and relaxing muscles in many organs, which is why it is used for irritable bowel syndrome and cramps. In Australia, indigenous people have exploited their continent’s Solanaceae’s Duboisia plants (corkwood) for centuries. These plants are also a source of scopolamine and hyoscyamine. The structure of the natural alkaloids has also inspired synthetic derivatives such as benzatropine, which is used to counteract the side effects of antipsychotic drugs and also for Parkinson’s disease.

Benzatropine is a synthetic analogue of atropine. It’s used to counteract dystonia, a side effect of prolonged use of many antipsychotic drugs.

Since atropine, scopolamine and hyoscyamine can easily be absorbed through the skin, it has been told that the flying ointment of underwear-less witches was rubbed on broomsticks, allowing their exposed rectums and vaginas to serve as absorption sites for the drugs. It’s the basis for the notion that witches “flew”— in reality hallucinating—on their broomsticks. When I first heard that story in my youth, I became forever interested in the combination of botany and chemistry!


The Botanical Garden. Roger Phillips and Martin Ryx. 2002

Tomato Glycoalkaloids:  Role in the Plant and in the Diet. J. Agric. Food Chem. 2002, 50, 21, 5751–5780

Tropane Alkaloids: Chemistry, Pharmacology, Biosynthesis and Production. Molecules. 2019 Feb; 24(4): 796

Structures from Chemspider.com