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Well, I am a bit confused. Plants do perspire and release water vapour but they also usually release heat (they have a metabolism and show on infrared).

It is a bit complicated. ;-)

It's true that plants give off IR radiation. But they also release water vapor, and the evaporation process is endothermic. This cools the plant tissues slightly, and conductance cools the air at the leaf surfaces. There's a lot of energy conversion and transfer going on around a functioning leaf. The "bottom line" of it all is that the air among masses of plant life is usually a few degrees cooler than the outside air, unless the air is cold, when the plants may somewhat warm the air. You can feel this when you walk into a clump of trees, even if you're still in the sunlight. Much of this effect is due to inefficiencies in the plants' techniques for controlling their own internal temperature.

It is interesting that plants can be giving off IR while being cooler than their surroundings. Part of the explanation is that the photosynthetic process involves a lot of frequency shifting. Photons are absorbed at one frequency, electrons bounce the absorbed energy around a bit, and another photon is radiated at a lower energy level. Most of this is significant to the plant's metabolism, but there are inefficiences. Thus, chlorophyll absorbs best in the green/blue part of the spectrum; a quick google found a graph at http://www.statemaster.com/encyclopedia/Chlorophyll [statemaster.com]. Note the complexity of the graph, with a lower peak in the red. To increase its absorbency, chloroplasts surround the chlorophyll with frequency-shifting molecules that absorb photons at other frequencies and reradiate the energy as photons that the chlorophyll prefers. But chlorophyll molecules don't intercept all these green/blue(/red) photons, explaining why leaves are greener than the incoming light. The whole process is impressively complex, and we're not very close to fully understanding it all. But much of the accidental reradiation is at low-energy frequencies, in the IR part of the spectrum.

Some interesting research reports a few years ago involved some tiny temperature sensors that could measure the temperature inside leaves. They reported that a wide variety of plants tested, over a wide range of atmospheric conditions, the internal leaf temperatures were close to 21 Celsius. This is somewhat cooler than our body temperature, and generally different from the air. The "higher" plants seem to have evolved some impressive temperature regulation methods, presumably because chemistry is simpler and cheaper if you can control the temperature. So at cooler temperatures, leaves tend to absorb lots of photons that they don't need for photosynthesis, but which function solely to warm the leaf to its operating temperature. The leakage from this process mostly loses low-energy photons, i.e., infrared. At higher temperatures, leaves can both radiate more energetic photons, and also release water vapor, which cools the tissues rapidly. In this case, most of the inefficiency is in heat absorbed from the surrounding warmer air, which is the main reason that clumps of plants are cool. But it's not really that the water was vaporized to cool the air. It was vaporized to cool the internal leaf tissues, and the air cooling is due to poor insulation at the leaf surface. The plants are trying to keep themselves at operating temperature, and cooling of surrounding warmer air is an inefficiency in this process.

Anyway, it's complex. And it's impressive how much control can be done by critters that have no muscles or nervous system and are stuck spending their whole life in one spot. It's almost entirely complex chemistry, including some very sophisticated control of photons and passing energy via electrons along chains of carbon atoms. Understanding what's known of it takes years of study. But you can find a lot of summaries by googling for someth