Photographers are generally fascinated by droplets of water and the interesting effects that are associated with them, including refraction, reflection and magnification. Often droplets result simply from rainfall or dew collecting on leaves. However, the perfect drops seen on the ends of grass blades after a cool, clear night, and on the margins of many other plants, have a more interesting explanation.

The process of guttation causes drops of xylem sap to appear on the tips of leaves of some vascular plants. On the surface of leaves there are stomata or pores through which water is lost by transpiration. At night, the stomata close, causing a reduction in transpiration. Drops of water are then forced out of the leaf through special stomata or hydathodes, which are found along the edges of the leaves or at the tips. It is believed that guttation is caused by high root pressure.

Guttation usually happens at night because, by day, water loss from the leaves is normally sufficient to maintain a negative pressure in the xylem. Conditions favoring guttation include moist soils, clear skies and light winds,which allows night temperatures to drop, thus raising the relative humidity.

I went out early this morning to Castor Hanglands, when the sky was clear and the temperature was still low, and spotted these guttated droplets on the leaves of hairy St.John's-wort. There was also plenty of birdsong, including the first tentative notes of the nightingale, not yet fully warmed up for its eventual glittering performance. As the sun warmed the atmosphere I watched five buzzards spiralling up on a thermal,until they rose so high that I couldn't see them at all. And on the way home I saw the first swallow - but it's not quite warm enough for summer yet!!

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