Why do Cypress Trees have Knees?
Some plants have the feature of growing aerial roots. They are either growing towards gravitational force or away from gravitation. There are various types of aerial roots that have several purposes.
Prop roots as seen in maize give support to its stem and stranglers as seen in banyan trees grow downwards and eventually form a fused strong pseudo trunk. This banyan tree grows as epiphyte on a host plant and gradually kills its host after proper development of stranglers. Pneumatophores are aerial roots that help the plant to breathe in soils that are water clogged as seen in black mangroves. Haustorial roots are found in parasitic plants like Mistletoe, and propagative roots named as stolons are found in strawberry plants.
Cypress trees which belong to the plant family Cupressaceae or cypress family are conifers. They live mostly in swamps. These trees also possess aerial root like structures called ‘Cypress knees,. These knees are vertical bulged parts that are grown on lateral roots. These roots are aerial and they appear conical, ranging from a few inches to a few feet in height. They are not considered to have developed as branches of the roots as they are not formed from the growth of the root tip primary meristem.
Then, how are they formed and how are they scientifically described?
For an unknown reason, the meristematic tissue called ‘cambium, abnormally divides in certain areas on the surface of the roots. This abnormal and exceptional development in the thick outer layers of Xylem ,one above the other every year, creates a large protrusion after a certain period. Scientists say that knees are not developed deep in the waters but at a certain height above the water. These projections or knees were compared with that of pneumatophores which exchange gas with the roots that are submerged. According to Betts (1943) knees serve as anchors as well as organs to supply oxygen to the roots. This was proven by other scientists like Schimper (1903) and Wells (1932) through certain experiments.
However, in some experiments where the knees were removed, the roots of these trees that dwell in swamps survived well. So ,some scientists concluded that these projections are meant for giving support to the plants as these trees tend to be weak in water.