To understand what is hidden behind a “four-leaf” clover, you must first understand what a leaf is. The leaf is one of the pieces that make up the basic vegetative anatomy of a plant , along with the root and stem. It is also the structure from which, by modifications of different types, many others are developed. Sepals, petals, stamens and carpels are, evolutionarily speaking, modified leaves.
The parts of the leaf
In the anatomy of a leaf we find several parts. The first, which partially surrounds the stem, is the sheath . It is sometimes bordered by two small pieces, the stipules . Then there is the petiole , normally elongated, which ends at the leaf blade , the broadest part of the leaf. Nerves are usually found in it, which may have different arrangements, although we often find a main one, in the center of the blade, which runs its entire length from the base to the apex , the final end of the leaf.
However, not all leaves retain that structure. Some are simpler and do without some of the parts. For example, the Ginkgo biloba leaf does not have a central main vein , but the veins divide in two as they approach the edge of the leaf blade. Another highly modified leaf is the vine tendril, which does not even have a blade.
On other occasions we find the opposite, much more complex leaves. This is the case of compound leaves . In them, the leaf blade is divided into subunits, called leaflets , which are articulated on what was evolutionarily the central nerve, which is exposed and is called the rachis . Each leaflet may also have its own small petiole, which is called a petiole. When the leaflets are arranged along the rachis, the leaf is said to be pinnate.
You can make things even more complicated, and instead of having a single tier of divisions, have two or even three. That is, a central rachis, which in turn divides into secondary rachis, which divide into tertiary rachis, along which the leaflets are arranged. This is what we find, for example, in carrot leaves.
But not all compound leaves have their leaflets along the rachis. In some species all the leaflets arise from the same point, as in the hemp leaf. We call this arrangement fingered. And when the number of leaflets is three, trifoliate.
The four leaf… clover
Clover leaves are normally trifoliate. The usual thing is that a clover does not have three or four leaves, but many more, and that each one is divided into three leaflets. The leaf has a long petiole, and is attached to the stem, which is usually creeping, with its sheath. The header photograph is therefore not a four-leaf clover, but a single clover leaf, which has four leaflets instead of three.
It is a curious rarity. It is estimated that only one in 10,000 clover leaves has four leaflets. However, it is an anomaly with high commercial and ornamental value. People still believe that this strange leaf with four leaflets will bring them luck.
But there are even rarer clovers. Some have leaves with five, six, seven… According to the Guinness Book, the clover leaf with the largest number of leaflets was discovered by Shigeo Obara in Japan in 2009, and it had 56 . Of course they incorrectly called them “leaves”.
In many plants it has been found that certain mutations alter the morphogenesis of the leaf, that is, the development of its shape. However, in the case of clover, it is thought to be due to a phenotypic response to different forms of environmental stress, which is not genetically fixed. In other words, certain disturbances suffered by the plant during its growth are what determine that its leaves develop more leaflets than usual. In fact, there are patented methods for its use in cultivation.
In 2009, a group of researchers from the Korean Society for Plant Biotechnology, led by In-Ja Song, irradiated clover seeds with gamma rays in order to obtain highly prized varieties. They achieved impressive results, with a first generation of almost 60% leaves with four leaflets, and even more, almost 20% with five. However, obtaining a second generation from these first results, they found that, as expected, there was no pattern of inheritance according to Mendelian genetics. The researchers concluded that the radiation would likely have hypersensitized the plants to environmental stressors .
Lee, W.-K. 2007. Variety of four-leaf clover and a method for breeding the same(Patent N.o US20030182693A1).Obara, S. 2009. Most leaves on a clover. Guinness World Records. https://www.guinnessworldrecords.com/world-records/most-leaves-on-a-clo verPérez Morales, C. 1999. Morfología de espermatófitos. Ed. Celarayn.Song, I.-J., Kang, H.-G., et al. 2009. Breeding of four-leaf white clover (Trifolium repens L.) through 60Co gamma-ray irradiation. Plant BiotechnologyReports, 3(3), 191-197. DOI: 10.1007/s11816-009-0091-x