Water plants in the wild
Water plants in natural conditions. Fortunately today you'd never be offered aquarium plants directly removed from their natural setting. In fact, in the U.S.A. our Dept. of Agriculture bans the importation even of cultivated plants. But in tank and pond culture plants retain many of the features they originally evolved in order to deal with features of their native ecosystems.
A yearly "monsoon" cycle of wet and dry seasons is characteristic of broad regions of the tropics and controls the natural cycles of fish and water plants and other organisms. Erosion that fills watercourses with silt is often a wet-season phenomenon; white water rivers, the hallmark of constant erosion, have their origins in young mountains, like the Andes. When their headwaters rise in old rock formations instead, undisturbed natural tropical waters are often very low in nutrients.
Plants control nutrient availability in the water by quickly scavenging any nutrient ions as they become available. So a low nutrient load is a limiting factor on water plants. With any flush of nutrient into the water, plants and algae quickly take up the slack. Low availability of nitrogen would favor nitrogen-fixing cyanobacteria. Planktonic diatoms can deplete silica from the upper sunlit level (epilimnion) and generate conditions that favor the growth of algae.
Water plants succeed where their particular form and metabolism continues to be encouraged by the seasonal conditions with which they co-evolved. Perhaps the familiar melt of Cryptocorynes, which seems to be triggered by any abrupt change in water and light conditions, favors the survival of Cryptocoryne species in a monsoon climate. Seasonal cold rains far upstream result in an annual flush of cooler water, washing silt with it. Once the monsoon spate has begun, light conditions for the next period of weeks will not favor photosynthesis in submerse leaves, and flood currents acting on leaf blades could pull plants free of the bottom and wash them away downstream. So perhaps there is survival value for plants that release their leaves at the first sign of a change; certainly there are many "Cryptocoryne streams" in South East Asia where there are virtually no other water plants.
Many floating plants stabilize themselves by hanging down trailing roots that act like ballasted keels. Caught in a riffle and sent spinning down into the water, a tiny leaf of duckweed will right itself. As water levels recede, floating plants may run aground and root themselves into an auspicious muddy location, then revert to an underwater leaf structure as waters rise again. Ceratopteris, or Water Sprite, behaves like this. Aponogeton bulbs may go dormant in damp mud during the dry season. Other more purely floating plants have evolved to have no real upper side: howsoever Elodea or Cabomba may roll in a current, they always present generous photosynthesizing surfaces to the light.
Other seed-bearing plants have taken only part way to the waters again, and never completely lose that waxy cuticle that first enabled plants to conquer dry land. Anubias is an example of such an amphibious plant. When Cryptocorynes or Sword Plants are grown emerse, they develop less translucent and stiffer cuticle-protected leaves.
I describe these forces acting on our aquarium plants in the wild because we are always more successful with plants if we keep in the back of our minds the basic strategies that lie behind their varying morphology. In 2003, an English translation of Christel Kasselmann's Aquarium Plants became available in the US. The introductory material alone has more extensive and detailed information on the ecology of aquarium plants in their native waters than has ever been available to aquarists before; there are evocative ecotope photographs, and every species entry has copious ecological notes. If you're curious about aquarium plants in their natural surroundings, this is a book to acquire.