SENSITIVENESS OF THE SUNDEW. 

 />)' F. H. Know It on 



THE insect-catching- habits of the delicate little sundew are, since 

 the researches of Darwin, well and widely known. The man- 

 ner in which these captures are made is very interesting. The 

 leaves are provided on the upper surface with numerous, in 

 some cases several hundred, little hairs, called tentacles, each tipped 

 with a tiny drop of a viscid secretion. The glistening of these drops 

 in the sunlight is what gave rise to the common name of sundew for 

 the plants. 



Little insects on coming in contact with the gland-tipped tenta- 

 cles are caught by the sticky secretion andjn their struggles to get 

 free excite the tentacle, which immediately begins to bend toward the 

 center of the leaf. Other nearby tentacles are also affected and are soon 

 pressing from all directions against the captured insect, being trans- 

 formed into a sort of temporary stomach, within the embrace of which 

 the insect is more or less dissolved and absorbed. The tentacles then 

 slowly regain their respective positions. 



It was found by experiment that the tentacles could be excited by 

 numerous other substances, some of which — as bits of meat or egg — 

 were dissolved, while others — as pieces of glass and so forth — were re- 

 jected. This selective action of the leaves was taken as a guide by 

 Mr. Darwin and many experiments tried with other substances, with 

 the result that various salts of ammonia were found to be most pow- 

 erful in stimulating movements. The small quantity of some of these 

 necessary to excite movement is truly wonderful. Thus a minute drop 

 containing r-960 of a grain of carbonate of ammonia placed on the 

 center of the leaf was sufficient to cause the marginal tentacles to be- 

 come inflected. A tiny drop containing 1-14,400 of a grain when 

 placed on the gland was sufficient to cause that tentacle to move, and 

 so small an amount as 1-268,800 of a grain when absorbed by a gland 

 was found sufficient to cause inflection. With nitrate of ammonia a 

 still smaller amount, namely the 1-691,200 part of a grain, was suffi- 

 cient to excite each tentacle into movement, and with phosphate of 

 ammonia the results obtained almost pass the bounds of possibility. 

 When a leaf was immersed in a solution of this salt of such strength 

 that each gland could only absorb the 1-19,760,000 of a grain, it was 

 enough to excite the tentacles to movement and even caused the clos- 

 ing of the whole leaf in some instances. That is to say this plant is 

 so exceedinglv delicate as to be affected by one nineteen-millionth part 



