CLIMBERS, PARASITES, SAPROPHYTES. 211 



Most insectivorous plants have enough chlorophyll to 

 enable them to make all the organic food they need, and 

 they can grow quite well when not supplied with insects. 

 When fed with insects, raw meat, or boiled egg, however, 

 the plants become stronger, flower more freely, and produce 

 stronger and more numerous seeds. Most insectivorous 

 plants grow in poor swampy soil, which is usually deficient 

 in nitrates and other available nitrogen-compounds. By the 

 capture and digestion of insects they obtain supplies of nitro- 

 genous food independently of the soil, and can in this way 

 grow in localities which would otherwise be unsuitable. 



Experiments with Sundew. Select leaves which have large 

 drops of the secretion on the tentacles, avoiding the dark-red leaves 

 which are too old to show the movements well. 



(a) Carefully place a very small fragment of raw meat on one of the 

 long outer tentacles, and watch the bending inwards of the tentacle by 

 which the meat is brought towards the centre of the leaf. Do any of 

 the other tentacles bend towards the piece of meat ? 



(&) Carefully dust a little powdered chalk over the gland of a 

 tentacle. In most cases no movement occurs, since the particles only 

 touch the secretion and not the surface of the gland itself. Place on 

 another tentacle a small solid body, such as a small particle of sand, 

 heavy enough to sink through the secretion and touch the tentacle ; 

 does this produce any movement ? 



(c) Strike the gland of another tentacle with a match-stick. Note 

 that a single tap does not produce movement. After watching the 

 tentacle for about a minute, tap it repeatedly and see whether move- 

 ment results. 



(d) Cut off a few leaves and place some in distilled water, others in 

 water made milky by stirring into it some powdered chalks. After 

 about 5 minutes, notice that the leaves in the milky water show bend- 

 ing of the tentacles, while those in the pure water show none or very 

 little. Vary this experiment by filling a narrow glass tube with water 

 and letting drops fall on a gland for about a minute ; no movement 

 occurs, but if water containing chalk is used, the tentacle bends 

 inwards. Hence the tentacles are not sensitive to pure water, but are 

 sensitive to water containing the finest solid particles. 



(e) Place a minute piece of raw meat in the centre of a leaf ; the 

 outer tentacles all bend in towards it. On another leaf place a piece 

 of meat about half way between centre and edge of the leaf, and note 

 that the tentacles bend, not towards the centre of the leaf, but towards 

 the piece of meat. Hence a stimulus received by any tentacle is 

 transmitted to the other tentacles. 



