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Alfred Russel Wallace

Alfred Russel Wallace (1823–1913) was a British naturalist, explorer, geographer, anthropologist, and biologist. He is best known for independently proposing a theory of natural selection which prompted Charles Darwin to publish on his own theory. Wallace did extensive fieldwork, first in the Amazon River basin and then in the Malay Archipelago, where he identified the Wallace Line that divides Indonesia into two distinct parts, one with animals more closely related to those of Australia and the other with animals more closely related to those found in Asia. He was considered the 19th century's leading expert on the geographical distribution of animal species and is sometimes called the "father of biogeography". Wallace was one of the leading evolutionary thinkers of the 19th century who made a number of other contributions to the development of evolutionary theory besides being co-discoverer of natural selection. These included the concept of warning colouration in animals, and the Wallace effect, a hypothesis on how natural selection could contribute to speciation by encouraging the development of barriers against hybridization. (more...)

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Guttation

An example of guttation, the appearance of drops of xylem sap on the tips or edges of leaves of some vascular plants, on an Equisetum. At night, transpiration usually does not occur because most plants have their stomata closed. When there is a high soil moisture level, water will enter plant roots, because the water potential of the roots is lower than in the soil solution. The water will accumulate in the plant creating a slight root pressure. The root pressure forces some water to exude through special leaf tip or edge structures, hydathodes, forming drops. Guttation is not to be confused with dew, which condenses from the atmosphere onto the plant surface.

Photo credit: Luc Viatour

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