iPhoneOgraphy – 10 Nov 2016 (Day 315/366)
In vascular plants, the root is the organ of a plant that typically lies below the surface of the soil. Roots can also be aerial or aerating, that is growing up above the ground or especially above water. Furthermore, a stem normally occurring below ground is not exceptional either (see rhizome). Therefore, the root is best defined as the non-leaf, non-nodes bearing parts of the plant’s body. However, important internal structural differences between stems and roots exist.
Aerial roots are roots above the ground. They are almost always adventitious. They are found in diverse plant species, including epiphytes such as orchids, tropical coastal swamp trees such as mangroves, the resourceful banyan trees, the warm-temperate rainforest rātā (Metrosideris robusta) and pōhutukawa (M. excelsa) trees of New Zealand and vines such as Common Ivy (Hedera helix) and poison ivy (Toxicodendron radicans).
Aerial roots may receive water and nutrient intake from the air. There are many types of aerial roots, some such as mangrove aerial roots, are used for aeration and not for water absorption. In other cases they are used mainly for structure, and in order to reach the surface. Many plants rely on the leaf system for gathering the water into pockets, or onto scales. These roots function as terrestrial roots do.
Most aerial roots directly absorb the moisture from fog or humid air.
Some surprising results in studies on aerial roots of Orchids show that the ‘Velamen’ – the white spongy envelop of the aerial roots, are actually totally water proof, preventing water loss but not allowing any water in. Once reaching and touching a surface the Velamen is not produced in the contact area, allowing the root to absorb water like terrestrial roots.
Many other Epiphytes – non-parasitic or semi-parasitic plants living on the surface of other plants, have developed cups and scales that gather rainwater or dew. The aerial roots in this case work as regular surface roots. There are also several types of roots creating a cushion where a high humidity is retained.
Some of the aerial roots, especially in the Tillandsia genus, have a physiology that collects water from humidity, and absorbs it directly.
The fossil record of roots – or rather, infilled voids where roots rotted after death – spans back to the late Silurian. Their identification is difficult, because casts and molds of roots are so similar in appearance to animal burrows. They can be discriminated using a range of features.