If an aquarium is to fulfill its esthetic function, it must be brightly illuminated. Artificial light is an absolute necessity for the development of plants. It is even more important than the quality of the water.
The length of the day changes in the temperate zone with the change of seasons. In spring the day is as long as the night on March 21. Then the days get longer and are at their longest on the first day of summer (June 21). The shortest day of the year is the first day of winter (December 21). On this day the night lasts 16 hours and the day only 8, During summer the days are essentially longer than the nights. In the tropics the length of a day and a night are equal, each of them lasting about 12 hours. In the temperate zone plants have in consequence much more light than in the tropics when totalled for the year. We therefore say that temperate plants are long-day plants while tropical plants are short-day plants. Only during the time of the spring and autumn equinoxes do our aquariums have a tropical day. For that reason tropical plants usually put out flower buds and flower by slow growth one to three months later. Tropical plants do not mind a longer period of light than they are accustomed to; it even helps speed their growth and the development of roots and leaves.
A long day in the temperate zone does not allow tropical plants to flower, since it acts as a brake on development of the sexual organs. On the other hand, they do not thrive well on a short winter day of 8 to 10 hours. They lose their leaves and some of the more sensitive plants may die.'If we want the aquarium to be decorative all year long we must prolong the light period during the winter. The plants require a day of at least 12 hours every day of the year.
The most important principle of aquarium lighting is that the duration of the light is more important than the intensity if everything else is constant. While it is not of drastic importance whether an aquarium is lit with 40, 60, or 100 watt bulbs (presuming the
minimal lighting intensity for the plants is reached), it is essential to consider for how long and at what time they are lit. It has already been stated that tropical plants are short-day plants and require light for 12 hours a day. The necessary hours of light cannot be replaced by more intensive light sources operating for a shorter period of time. It is not necessary to measure the intensity of the light in lux (a unit of illumination equal to 1 lumen per square meter; a lumen is the amount of light which falls on an area of 1 square centimeter placed 1 centimeter from a standard candle) as long as the plants can easily become accustomed to weak or strong light, but tropical plants cannot get used to too short of a day.
A lightmetercan be of great help to those who would like to monitor the light requirements of their plants. Photo by Dr. H.R, Axelrod.
By additional artificial light we mean the prolongation of daylight in winter, not the increase of the intensity during the natural day. In practice this means that in winter it is necessary to prolong the plants' day by about 4 hours. It does not matter whether this period of additional light is given in the morning before dawn or in the evening after dusk. It also does not matter whether the 4 hours is given as a unit or divided between morning and evening. In order to keep the plants on a regular rhythm or schedule, the light should be given at the same time each day. Thus the day should not be prolonged in the morning one time and in the evening the next,
If an automatic time switch is available, this greatly simplifies the keeping of a regular lighting schedule. The extra lighting can be turned on automatically at, say, 3:30 p.m. and kept operating until at least 8:00 p.m. If the light can be kept on until 10:00 p.m., so much the better. Plants kept under this schedule will have the same growth rate all year long and will develop as well during the winter as during the summer.
Plants react best to light which is predominantly in the red and blue wavelengths. Such lighting is readily available in the form of special fluorescent tubes, such as the Gro-Lux type, sold in most aquarium and plant shops.
The importance of the length of daylight is often underestimated by aquarists, and we are convinced that failures with some sensitive tropical plants are caused largely by short winter days.
A collector pulling out a Cryptocoryne from its shady natural habitat in a small stream in Singapore. Photo by Lee Chin Eng.
DO SHADE-LOVING PLANTS EXIST?
Many experts believe that shade-loving plants do not exist. Among aquarium plants the cryptocorynes have the most familiar reputation as shade-loving plants. It is true that in nature most of them grow mainly in shaded situations where no other low-growing vegetation exists. Do they live in deep shade because it meets their requirements best or because they have been forced there through competition? In our opinion there is no doubt that they have been forced into the shade.
The soil in the tropics is very poor, and plants which grow rapidly do so due to their strong root system. Cryptocorynes have a relatively weak root system and are not able to extract the needed nutrients from the soil. In a natural tropical society of plants cryptocorynes are not able to compete with other, more aggressive spe cies which can outgrow and suffocate them. For this reason cryp-tocorynes have had no other "choice" than to colonize the most shaded situations where competition does not exist. They have a great ability to survive in subdued light. Only in artificial culture can we see how much some cryptocorynes love sunlight and how well some species do under its direct rays.
The same applies to their submersed forms, but only if given the right quality of water. It is not a question here of shade-loving plants but of water in shady areas. In a permanent half-shade or in permanent artificial light water holds its good qualities. If water is alternately sunlit and shaded, sudden changes take place. There are changes in the number and species of protozoans, algae, and bacteria. In consequence both chemical and physical qualities of the water change and these changes effect a negative growth in the development of plants.
Even if the above explanation is nearer to the real situation than most other explanations, nothing changes the fact that the best position for an indoor aquarium is a place away from the window and sunlight. The plants will improve with the assistance of artificial light. The important thing is to prepare the most constant and favorable conditions of development throughout the year. Do not be afraid of plenty of light even with "shade-loving" plants, but protect your aquarium from undesirable fluctuations of light intensity. It has been reliably demonstrated that many shade-loving plants easily become accustomed to a much stronger intensity of light and that plants occurring in nature in constantly lit localities usually become easily accustomed to a constant half-shade in the aquarium. Exceptions will be mentioned with the descriptions of the individual species of plants.
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