Zindaginha - I think you nailed it with your discovery that orchids are different than everything else... Orchids have evolved in ecological niches where they didn't have to compete for light and space (trunks and branches of trees) and also where they were safe from herbivores. In a tree there is not a consistent source of water - it rains and then it stops and the sun comes out, so they had to evolve mechanisms and structures to conserve water. They also had to manage on very minimal nutrients - like the organic matter from detritus above them that dissolves in rain. One of the ways in which they manage that is to grow slowly. (A tomato plant may grow a foot (1/3 m) a day, where an orchid may add one growth a year... fertilizer requirement is proportional)
Because of the huge variations in those niches that orchids occupy (side of tree, horizontal branches, shade for those below the canopy and sun for those above, etc) speciation is very complex. But if you look at the detail of the environments in which different orchid species grow, their requirements start to make sense.
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