202 THE BIOLOGY OF FLOWERING PLANTS 



and high atmospheric humidity — many plants exude drops 

 of liquid water — a phenomenon sometimes referred to 

 by the inelegant term " guttation." The drops of water 

 seen on the tip of every blade of grass after a close, damp 

 night are not dew but drops excreted at the leaf tips. The 

 process may be easily demonstrated by placing a pot of oat 

 seedlings, 4 or 5 in. high, under a bell jar in a warm room, 

 and watering thoroughly. Drops begin to appear on the 

 leaf tips after a few minutes and, if removed, quickly reform. 

 The excretion is active and abundant. It is due to a rapid 

 supply from the root, forced up under root pressure : when 

 evaporation is high transpiration can cope with the supply, 

 when it is low the water is forced out as a liquid. 



In many leaves, common examples being the fuchsia, 

 the garden nasturtium and the balsams, special water- 

 excreting stomata are formed at the tips of the serrations. 

 These stomata may occur singly or in groups. They are 

 commonly large and permanently open. Below them lies 

 a loose parenchymatous tissue, with abundant intercellular 

 spaces, called the epitheme, and below this lie the terminal 

 tracheids of a vein, spread out in a brush. The whole 

 organ is called a passive hydathode, passive because the 

 water is forced through it from the tracheids. Epidermal 

 hydathodes which, acting as glands, actively excrete water 

 are also known. 



Such hydathodes occur and function on leaves of water 

 plants ; they have been found on the lower surface of float- 

 ing leaves and on submerged leaves, and no doubt act in 

 eliminating the water raised by the transpiration current. 

 They are also frequent on the leaves of land plants, and in 

 particular of plants of humid tropical regions. As Schimper 

 writes, " Early in the morning, especially in the tropics, 

 many plants, herbs as well as trees, are so covered with 

 drops of water that not infrequently a drizzling rain seems 

 to be descending from the forest canopy of leaves." 

 According to F. Shreve (191 4) hydathodes are infrequent in 

 the Jamaican rain forest. The amount of water given off 

 may be quite large ; the most celebrated case known is that 



