Wednesday, 8 March 2017

How to be heat and drought resistant?


The Oleander has big, red, white or yellow, highly fragrant flowers. The 7 - 8 m high shrubs are spread from the Mediterranean to East Asia, growing in sunny locations and near watercourses. In North West Europe it is often drawn as a container plant.

Its leathery, lance-shaped leaves show the characteristics of xeromorphic - that means drought resistant - sun plants. Oleander is adapted to the very hot and dry borders of watercourses in summer.
A multilayer epidermis with a thick cuticle covers the leaf upper and bottom side. Underneath the top side lies a double row of palisade parenchyma. At the bottom side of the leaf, is a multi-row layer of palisade like cells with parietal chloroplasts. Between the two lies the loose spongy parenchyma with numerous, nearly parallel aligned vascular bundles. The latter stiffen the leaf. 


Deep cavities, each containing multiple stomata and numerous hairs, break through the palisade layer at the bottom side of the leaf, reaching the spongy parenchyma. These provide a water vapor-filled, sheltered space, whereby the evaporation out of the leaf is being reduced. The cavities are very close together, therefore the leaf has many slit-like openings. These enable the leaf to assimilate intensively in humid conditions. In dry conditions however, they prevent evaporation by closing themselves. Oleander has sun leaves which are protected against evaporation.

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