The plant kingdom is full of curious examples of plants that leave a precise record of their life histories somewhere in their preserved tissues. The most familiar examples of these sorts of "logbooks" are the annual rings in the wood of oak trees. These annual rings have been used to date Viking fortifications with a precision of one year!Less well-known examples include aquatic plants such as the seagrass Cymodocea, which has been used to reveal submerged sand-dune movements in Mediterranean coastal areas. Another seagrass Thallasia, has been used to measure the amount of sand deposited after hurricanes in the Mexican Gulf. Both seagrasses leave leaf scars on the shoot and by measuring the distance between these scars one can determine the height of the sand deposited or removed.
Recently, I discovered another example of these quaint life-history markers. Petiole and inflorescence scars on the rhizome of the Yellow Waterlily. In a manner similar to most life-history markers in the plant kingdom, it is caused by the seasonality of growth events. Waterlilies bloom only during the summer but produce leaves all year long. The "marker" turns out to be that flower scars and leaf scars leave distinct and different scars on the rhizome.
But first some more details of the life history of the Yellow Waterlily. It starts as a seed which settles on the lake or pond bottom. If the physical and chemical conditions are suitable, the seed will germinate. For quite a while however, the plant will not look like the waterlily that we are used to seeing.
The tiny seedling will remain as an obligate submerged plant for a long period (years) until enough starch has been stored in the rhizome so that a floating leaf can be sent to the surface in the spring. The exact period of submerged growth probably depends on a variety of factors including the depth at which the seedling grows, the transparency of the water and thereby the light climate at the particular water depth. It is also likely that the sediment’s fertility plays a role. When the first leaf emerges and reaches the water’s surface the leaf soon starts to add to the total energy budget of the plant. The floating leaf develops stomata and benefits from the greater availability of CO2 in air compared to water. Still, the energy available to the plant is not sufficient to enable the rhizome to create a flower. Flowers contribute little to help sustain a plant, but they require a lot of energy to make. They represent a withdrawal on the plant’s total energy budget. Perhaps, a few more years pass during which the rhizome produces emergent leaves during the summer and submerged leaves during the remaining part of the year.
Suddenly one summer the first flower emerges! Initially only one flower is thrown up per year but later, if the rhizome has experienced good conditions and is able to store a lot of starch, it may throw up two each year. This is where the dating technique comes in. Flower scars on the rhizome of the Yellow Waterlily are almost circular whereas the leaf scars are more oval. By simply counting the number of flower scars you can determine the age of the rhizome. But watch out! In some years the plant may have produced two flowers per summer. Happily, this is easily recognised by the horizontal placement of the scars. In addition, the number of proceeding seasons before the first flower event can be determined based upon the assumption that the number of leaves set per year is almost constant - count the number of leaf scars before the first flower and divide by the number of leaves set per year (known from the younger part of the rhizome) and you have the number of years proceeding the first flower!
All these dating exercises can only be done if the plant material, that holds the information of seasonality, is preserved for more than one growth season. These requirements are met for the Yellow Waterlily. The rhizome shown here is 60 cm long and at least 9 years old!
| Ole Pedersen | Ole Pedersen's Homepage |
 NetPets® Main Page |  The Fish Center |