09-22-2009, 04:43 PM
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Yes, kelly528 is absolutely correct. Land plants (with leaves in the air--emersed) and aquatic plants (leaves under water--submersed) have two very different structures to their leaves to facilitate the different methods of obtaining nutrients, and the nutrients themselves that they require are not identical. A true land plant, such as house plants and garden plants, cannot be submersed; if they are, they will die as kelly529 said.
Some aquatic plants are fully aquatic, living their entire lifespan under water (submersed). To my knowledge, all the stem plants are true aquatic plants. But many of the rooted plants such as several species of Echinodorus (swords) are true bog or amphibious plants in nature. They spend six months emersed (during the dry season) and six months submersed (during the rainy/flood season). They flower during the emersed period, and some have different methods of reproduction when submersed [runners with no flowers but daughter plants]. Not surprisingly, such plants have different leaf forms for these two periods.
This is why when you buy potted aquarium plants in the store and plant them in your aquarium, the leaves on the plant when you bought it usually die off within a few weeks, and the new aquatic leaves will look (sometimes considerably) different from the original leaf. Most nurseries raise aquatic plants emersed because it is quicker and less expensive, so the leaves on the plants are the emersed form. The difference in leaf shape is connected to the leaf functions, and being emersed or submersed requires different leaf structures to manage those functions. So, as I mentioned at the start, the leaves of land plants are different in their structure, and will not survive long under water. And the two types of leaves on amphibious bog plants is an illustration of this fact. Fortunately, the bog plants we keep in our aquaria are able to live totally in the submersed form; land plants cannot do this.