Several species of cut flowers that originate from the tropics are also grown in greenhouses in cooler climates, and some of these have become important commercial commodities, for example Anthurium and orchids. Anthurium andraeanum flowers are not sensitive to problems of water uptake, probably due to their low transpiration rate. The vase life of Cymbidium is also not limited by problems relating to water uptake. This is mainly due to the absence of stomata, which results in an extremely low rate of transpiration, compared to other cut flowers. The vase life of Phalaenopsis orchids, in contrast, is often short due to vascular occlusion. Other tropical flowers, in particular a number of monocots, for example Canna sp. and Heliconia psittacorum, have a vase life that is limited by inhibition of water uptake. In this group the blockage for water uptake is apparently mainly due to exudation of mucilage at the cut surface. Still other flowers originate in cool climates but can be grown in greenhouses in the tropics, for example roses. When grown in the tropics, roses tend to have thicker stems and often show earlier symptoms of water stress than flowers from the same cultivar grown in a climate with lower light intensity. A similar response is observed in chrysanthemums, where stems that are more woody show earlier symptoms of water stress. This is due to the earlier development of an occlusion, located in the stem base, that is large enough to inhibit the rate of water uptake to such an extent that it becomes lower than the rate of transpiration. In roses the occlusion is not due to plant mucilage, but to bacteria and xylem cavitation.