MICROSCOPE AT THE POND-SIDE. 61 



square, and attached together. The last form is 

 seen in Melosira, species of which are figured 

 at 36 and 37, plate 2, and 139, plate 5. The 

 most common forms are those which are oval, or 

 boat-shaped, and represented by species of Pin- 

 nularia and Navicula in figures 34 and 35 a, in 

 plate 2. Some of these are again larger at one 

 end than the other, as in Surirella, figure 33. 

 The markings upon the surface are very various. 

 In some forms the markings are exceedingly 

 minute : so small are they, that certain species 

 of diatoms have been used as test objects, for 

 testing the highest powers of the Microscope. 



Whilst living, the diatoms possess the power of 

 moving about, and in some of them, as well as the 

 desinids, a movement has been observed of the 

 small particles in their interior. The diatoms are 

 generally of a brownish or brownish-yellow colour, 

 which seems to be due to a small quantity of iron 

 in their composition. They are increased in the 

 yame way as the desmids, by the production of new 

 cells between the parent frustules. This process 

 is seen in figure 3o, a and &, in plate 2. The 

 continuance of the species in these organisms is 

 secured by the process of conjugation and the sub- 

 sequent formation of the spores. This process is 

 exhibited in figures 13o and 136, plate 5. In some 

 cases, however, the spore is found without the union 

 of two cells, as in Melosira represented at figure 137, 

 plate 5. 



Sometimes, attached to the bottom of a pond or 

 river, or growing from immersed objects, or floating 

 about in the water, will be found long green fila- 

 ments. These are the fronds of confervse. All 

 forms of these and they are very numerous will 

 be found most beautiful objects for examination. 

 They may be laid on a slip of glass in water, and 



