204 THE BIOLOGY OF FLOWERING PLANTS 



epidermal cells. He supposed this to lead to a rapid 

 spreading out of rain-drops to a capillary film which would 

 evaporate rapidly. A similar function has been assigned to 

 the covering of soft hairs shown by such plants as our 

 native Stachys sylvatica. Unwettable leaves — the surface 

 of which is coated with wax — avoid wetting altogether. 

 But that any of these features is of real importance in nature 

 remains to be proved. 



{e) A very striking character of pulvinate leaves is their 

 assumption of a more or less vertical position at night. We 

 have already seen that such leaves may become vertical 

 in intense insolation. The same position, due in this case 

 to an unequal increase of turgor, is assumed in the dark. 

 Less frequently light and dark positions are assumed by 

 non-pulvinate leaves, e.g. of Impatiens and Amarantus, as 

 the result of differential growth rates in the petiole. The 

 exact relation of these sleep movements, or nyctinastic move- 

 ments, to external conditions is very complex. In a simple 

 case like that of Acacia ccesia or Acacia lophantha the move- 

 ment, here a folding up of the pinnules, follows within a 

 few minutes of darkening, and the reverse opening movement 

 follows illumination. But a reverse movement also takes 

 place without illumination, whether as a delayed effect of 

 the darkening, or as a reaction to the previous movement 

 we do not know, though the latter is the more likely. In 

 Acacia these movements may be repeated as often as twelve 

 times in the 24 hours. The primary leaf of the scarlet 

 runner, however, carries out its double movement only 

 once in the 24 hours, and more frequent changes of light 

 and dark scarcely influence it. In constant darkness it 

 swings regularly in 24-hour periods for some days. Stoppel 

 (1912, 191 6) has shown that a seedling grown in uniform 

 darkness exhibits this daily periodic movement unmis- 

 takably. She has attempted to show that under these 

 conditions it is regulated by changes in atmospheric 

 electricity, but her results have been challenged by 

 Schweidler and Sperlich (1922), and by Cremer (1923), 

 and the question of causation is undecided. Between 



