Chap. XVn. STEUCTURE OF THE BLADDER. 397 



points. Each point is tipped by a short, straight 

 bristle ; and slight notches on the sides of the 

 leaves bear similar bristles. On both surfaces there 

 are many small papillae, crowned with two hemi- 

 6j)herical cells in close contact. The plants float 

 near the surface of the water, and are quite destitute 

 of roots, even during the earliest period of growth.* 

 They commonly inhabit, as more than one observer 

 has remarked to me, remarkably foul ditches. 



The bladders offer the chief point of interest. 

 There are often two or three on the same divided leaf, 

 generally near the base ; though I have seen a single 

 one growing from the stem. They are supported on 

 short footstalks. When fully grown, they are nearly 

 -iV of an inch (2*54 mm.) in length. They are trans- 

 lucent, of a green colour, and the walls are formed 

 of two layers of cells. The exterior cells are poly- 

 gonal and rather large ; but at many of the points 

 where the angles meet, there are smaller rounded cells. 

 These latter support short conical projections, sur- 

 mounted by two hemispherical cells in such close 

 apposition that they appear united; but they often 

 separate a little when immersed in certain fluids. The 

 papillae thus formed are exactly like those on the 

 surfaces of the leaves. Those on the same bladder 

 vary much in size ; and there are a few, especially on 

 very young bladders, which have an elliptical instead 

 of a circular outline. The tw^o terminal cells are 

 transparent, but must hold much matter in solution, 

 judging from the quantity coagulated by prolonged 

 immersion in alcohol or ether. 



* I infer that this is the case om Lentibiilariacese," from the 



from a drawing of a seedling ' Videnskabelige Meddelelser,' 



given by Dr. Warming in his Copenhagen, 1874, Nos. 3-7, pp. 



paper, " Bidi-ag til Kundskaben 33-58. 



