HYDATHODES: DRIP-TIP 203 



of Colocasia antiquorum, the taro, from the leaf tip of which 

 nearly 200 drops may be spirted off per minute, while a single 

 leaf may excrete 100 c.c. in a night. 



We have here an evidently widespread mode of 

 increasing the amount of water given off by the plant ; not 

 an actual increase of transpiration, but a supplementary 

 excretion. It is suggested that this is important because 

 it increases the supply of salts, particularly at night when 

 growth in the warm tropics may be rapid. It has also been 

 suggested that harmful substances may be got rid of in liquid 

 water. It has been further suggested that the excretion 

 prevents an injection by water of the intercellular spaces of 

 the leaf, which would of course hinder gas diffusion and 

 consequently assimilation. This last suggestion is at com- 

 plete variance with the other two, for it does not suppose 

 the water stream to perform any useful function ; excretion 

 merely gets rid of too great a supply of water. Decisive 

 experimental evidence is lacking. At present, then, we cannot 

 say whether excretion of water is of any use, or is simply a 

 consequence of excessive water supply. 



(c) A structural feature common among tropical plants 

 of humid regions, such as the rain forest of the Cameroons 

 and of West Java, is the elongation of the leaf tip into a 

 narrow point. This point, or drip-tip, is supposed, by 

 Stahl (1893), to facihtate the run-off of rain-water, so that 

 the leaf dries more quickly after rain and consequently 

 transpiration sets in again more quickly. As the diffusion 

 of carbon dioxide through the stomata will also occur 

 more quickly, this feature might equally well be regarded 

 as promoting assimilation. According to F. Shreve, 

 (19 146), however, the drip-tip has scarcely any effect in 

 hastening the drying of the leaf. He determined the rate 

 of drying for a number of leaves with elongated tips, for 

 the normal leaf, and for the leaf with tip removed, and 

 found practically no difference. This evidence seems 

 decisive. 



(d) Stahl (1893) also pointed out that many rain-forest 

 leaves have satin-like texture due to papillose structure of the 



