The great variety of the landscapes on our planet is due in no small measure to the fact that the vegetation indigenous to each region is suited to the climatic conditions likely to occur. It is no coincidence, for example, that the giant trees of the tropical rainforest are so different from the coarse grass of the cold, northern tundra. The plant-life of each climatic zone is that which is best suited to the local climate.
In general the stature and complexity of vegetation increases with the level of precipitation. Where water is plentiful, it is dense and lush; where moisture is in short supply, plant-life is sparse and stunted. A continuous spectrum can be identified, which runs from the arid desert - with no growth at all - to equatorial regions where the abundance of rain allows the tallest and most luxuriant of trees to thrive in the evergreen rainforests. In between, rainfall less than about 200mm per year is sufficient to support only grassland and shrub, while the temperate forests of northern Europe thrive in regions where annual rainfall is more than around 1,000mm.
But survival sometimes dictates that plants must further adapt to the subtleties of the regime in which they find themselves. In the dark rainforests of the tropics, for example, although warmth and moisture are present in abundance, sunlight is in short supply for many of the lower plants. They need sunshine to create the material for growth by photosynthesis, and they must take full advantage of every speck of light that filters through the foliage above. Their leaves act, as it were, like solar panels in this context, and even a thin film of rain can interfere with their efficiency; it reflects a large proportion of the light.
One solution to this problem is for the plant to evolve leaves of such a shape that the water drains away - leaves that are often equipped at the ends with downward-pointing spikes that act like eaves. Other plants secrete an oily coat of resin to repel the water; the rain either runs off the leaf immediately, or else accumulates in separated drops, rather than covering the surface in a film.
Another trick is for the leaf to cover itself with tiny hairs; they act as umbrellas, and prevent the drops from making contact with the surface. Indeed, in these circumstances, the drops can then act as little lenses, which by focusing the sunlight, dramatically increase - in certain spots - the light available to the plant for photosynthesis.