Temperature / Survival  - Trees grow in some of the hottest regions of the world, where other survival factors, especially water, are proper for growth.  Trees do not grow in regions of the world where temperatures are -40C and F (both are the same at -40) for long periods, usually for several weeks at that temperature or below.  Trees developed in warm climates.  More species of trees are in the forests of the equator, than in forests in the subtopics and temperate zones.  Some trees have developed unique ways to live in parts of the world where temperatures far below freezing occur, but not where -40C or F occur for long periods.  How do trees do it?  Details on how they do it are not well understood.  Wood will freeze, but the water is in the spaces between the living cells, and this prevents the living cells from bursting.  The tree may use avoidance tactics.  The water may be bound with other substances.  The cellular water may also be cleared of all microscopic particles that could act as nucleators.  If water is pure hydrogen and oxygen without any impurities or foreign microscopic particles, and if the water is kept absolutely still, it will not freeze until -40C or F.  At -40C or F, the water molecule becomes its own nucleator and sets off a rapid chain reaction.  Bacteria or their coatings may act as nucleators and cause ice to form quickly over leaves, and protect the inner tissues from further, slightly colder temperatures.  Plants that have no ways to respond to freezing temperatures die quickly when the water in the cells freezes and the cells burst.  But, some "cold injury" is not caused by cold but by wounds (see cracks). 


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