Terrestrial or Aquatic Plants?
A poor purchase can sometimes be the result of buying paludarium plants, as they need time to adapt. Transitioning a plant from a submerged form to an emergent (above-water) form is much easier than the reverse. If you suspect that a newly purchased plant is an emergent form (which often happens with young Cryptocoryne and Echinodorus), do not submerge and plant it directly into the substrate right away. Instead, let it float near the surface until one or two new leaves appear.
Echinodorus amazonicus – Emergent form Ammannia gracilis – Emergent form
When buying plants to decorate an aquarium, it is useful to know whether they grew in water or in the air. Any observant enthusiast can figure this out. What challenges do plants have to overcome on land? First and foremost, obtaining water and transporting it to all their organs. Aquatic plants are spared from this. Water surrounds them, and buoyancy is present. This is precisely why there are clear differences between them, noticeable even to the naked eye. You just need to take the plant out of the water.
Water is many times denser than air and naturally supports the plants living within it. This leads to the underdevelopment or absence of their supportive tissues, which is why the stems and leaves of many aquatic plants are soft, flexible, and easily moved by currents. Water flow, turbulence, wave action, and convection currents help supply plants with nutrients, heat, and air.
Organs of aquatic plants that come into contact with water can absorb water and dissolved oxygen across their entire surface. This is achieved through the special structure of their epidermal cell walls. Gas exchange is further enhanced by the highly dissected nature of submerged leaves, which increases their surface area in contact with water, as well as the presence of large intercellular spaces and air cavities in all organs for air transport.
The root system of aquatic plants is weakly developed, and root hairs are absent: water with dissolved mineral substances can penetrate directly into the leaves. The high density of the aquatic environment causes a weak development of mechanical elements in the leaves and stems of aquatic plants. The few mechanical elements present in the stems are located closer to the center, granting them greater flexibility. Roots are feathery since plants do not require structural support in water.
Aquatic plants have poorly developed or even absent vascular bundles. A characteristic feature of hydatophytes is the extreme fragility of their stems and leaves, which collapse when removed from water. Completely submerged in flowing or standing water that supports them from all sides, they do not need rigid tissue elements (sclerenchyma), and thus exhibit significant simplicity; parenchymal tissue (consisting of multi-faceted, delicate cells not elongated in any direction) forms the main mass, in which vascular-fibrous bundles are very poorly developed.
Echinodorus and Anubias grown in paludarium conditions.
Terrestrial plants are forced to form mechanical tissues to support their stems and leaves, preventing them from sprawling helplessly on the ground. In other words, they are sturdy and rigid—much more rigid than aquatic ones. When a terrestrial plant is pulled out of the air, water can be easily shaken off. Water forms droplets rather than a continuous film on the leaves. Leaves and stems of aerial shoots sometimes feature pubescence, which never occurs in submerged plants. Aquatic plants never have velvety leaves. Furthermore, underwater plants do not have thick, succulent stems and leaves that store water. They always have access to sufficient water around them at all times.
Even if the stems appear thick, they contain an internal system of air-bearing tissues rather than fluid, ensuring buoyancy and air transport underwater to the roots, much like water lilies (Nymphaea and Nuphar). Such stems feel soft to the touch and easily compress when pressed. A lack of protection against drying out is a universal property of all submerged plants. Their shoots, when removed from water, look truly wet. The delicate leaves, moistened by water, stick together. Cut terrestrial plants first wilt and then dry out into hay. In contrast, aquatic plants lose water very quickly and dry out right before your eyes. You must keep this circumstance in mind when handling them.
There is only one definitive way to know that a plant will never grow underwater: if it has thick, fleshy stems and leaves filled with juice that cannot be easily compressed between your fingers, especially if they are covered with a waxy coating. These plants belong to the group of succulents—inhabitants of arid regions. In their evolutionary path, they have moved very far away from water, conquering deserts and mountain regions, and adapting to scorching sun and prolonged droughts. They will not survive in an aquarium.
Many hobbyists try to adapt moisture-loving houseplants to aquatic conditions. It must be stated immediately that such plants, even if they adapt wonderfully to aquatic environments, cannot be kept fully submerged for a long period of time. Many of them can be kept in a half-submerged state on the side shelves of an aquarium or in a paludarium.
…
