1903 Nervous Mechanisms of Plants 325 



slowly inwards upon their prey, glued to the leaf by its 

 viscid surface. 



The effect of chemical stimulus is apparent in the 

 phenomenon of " chemiotaxis," in which we have certain 

 organs of plants, such as the archegonia of ferns and 

 mosses, secreting malic acid, cane-sugar, and other sub- 

 stances to attract the anthcro/.oids which come swarming 

 round, and in this way fertilisation is ensured. 



Sensitiveness, accompanied by slow and constantly re- 

 curring movements, is well seen in nyctitropism, or the 

 " sleep of plants," under the influence of light and variations 

 of temperature. We have already noticed that certain 

 plants, such as the clover and the wood-sorrel, close up 

 their foliage leaves at the approach of night or cloudy 

 weather, as well as some flowers close their petals. How 

 quickly some plants respond to the variation of temperature 

 accompanying the lessening of light is remarkable, as the 

 variation can be but slight, especially in the middle hours 

 of the day, when some flowers regularly close their petals. 

 Towards evening the temperature is decidedly lowered, and 

 several plants fold up their leaves as a protection against 

 excessive cold and moisture. The closing of flowers is 

 mechanically due to the greater or lesser turgescence of the 

 cells and the more rapid growth of the cells away from 

 the direct rays of light. We have also instances of the 

 sensitiveness of some leaves to intensity of light. Where 

 great heat during the day would have the effect of bleach- 

 ing or destroying the delicate chlorophyll, some plants turn 

 the edges of their leaves towards the direct rays of light. 

 Not only does the leaf in some cases assume a protective 

 attitude, but the chlorophyll granules are said to actually 

 move about the cells, taking up different positions in diffused 

 light, bright sunlight, and darkness. A remarkable example 

 of this " paraheliotropism " is to be seen in the cells of the 

 Ivy-Leaved Duckweed {Lemna trisu/ca), which grows so 

 freely in ponds and ditches. 



