30 THE BOOK OF NATURE STUDY 



bogs may be formed on other soil than peat, provided it is a soil 

 that does not allow the water to percolate through, as on Fuller's 

 Earth. The character of the vegetation in marshy and boggy 

 ground depends on whether the soil is peaty and the water rich 

 in mineral substances such as lime. The water of peat bogs is 

 poor in lime, and the plants are mostly slow growing and 

 dwarf y. A marsh usually has water rich in mineral substances, 

 and plants grow rapidly. Bogs may be found at any height, 

 from a few feet above the sea level to several thousands ; but a 

 marsh is more usually on low ground, on the outer margins of 

 ponds and streams and in hollows where drainage is deficient ; 

 it may even mark the position of a former pond which has been 

 filled up and drained. Plants living on marshy ground will be 

 superseded by other species if the ground is drained. This is 

 very well seen in deserted brick ponds. As the water gets less 

 and less, different plants begin to appear on the sides, which are 

 drier than they were when the pond was fuller ; by degrees 

 plants which do not care for a moist soil establish themselves, and 

 in a few years the vegetation may seem almost xerophytic. The 

 following plants were gathered on the slopes of a deserted brick 

 pond in April : Colt's-foot, Groundsel, Rockcress, Stinging Nettles, 

 Purple Deadnettle. There was very little water left in the pond, 

 and only some algae in it. What time has elapsed since that 

 pond was first deserted is not certainly known, probably at least 

 ten years. 



Insectivorous plants are characteristic of peat-bogs ; these 

 plants get the nitrogen necessary for their life from the insects 

 on which they feed. A great many experiments were tried 

 by Darwin, in order to ascertain whether these plants would 

 absorb non-nitrogenous fluids. He placed drops of distilled 

 water on the leaves of Sundew, but the tentacles which are situated 

 on the edge of the leaf remained motionless ; then he tried solu- 

 tions of gum arabic, sugar, starch, alcohol, even tea, but in experi- 

 ments on sixty-one leaves no effect was produced ; the tentacles 

 would not respond to the stimulus of non-nitrogenous liquids. 

 The case was very different with nitrogenous foods, such as milk, 

 albumen fresh from a hen's egg, saliva, isinglass, etc. In the case 

 of milk he found that the tentacles took forty-five minutes to 



