45'^ PHYSICAL GEOGRAPHY. 



containing water. A very little practice will enable the searcher after 

 diatoms to distinguish them, or to choose localities likely to yield them. 

 I have found that, if the adherent mud on the submerged wood-work of 

 a bridge or pier be scraped off and transferred to a bottle with some 

 water, and, when brought home, be placed in a saucer or plate, covered 

 with the water and exposed to the diffused sunlight which comes in at a 

 south window, many very beautiful forms may be procured in sufficient 

 quantity for observation ; and, besides, in such saucers the diatoms may 

 be kept and grown for a length of time, and many points in their econ- 

 omy studied with facility. Thus they may be watched through the 

 process of growth by subdivision and conjugation, and the changes 

 which they then undergo* observed without being under the necessity 

 of making several visits to their native localities to make collections. 

 The dead skeletons of many rare species are to be found in the muds of 

 our tidal rivers and harbors, that from some of our southern streams 

 especially, where the summer season of vigorous growth lasts longer 

 than with us, having yielded forms not otherwise procurable. All alga?, 

 as the water-plants which do not bear apparent flowers are called, both 

 marine and fresh water, bear upon their fronds diatoms in greater or less 

 numbers; and the results of dredgings in deep water will provide the 

 student with ample material for many an hour's amusement and instruc- 

 tion. The various methods to be employed, in preparing elean or mixed 

 gatherings of diatomaceaD, can for the most part only be learned from 

 experience, as the books tell us little on this subject. Some general 

 directions, however, on this point will be appropriate to this sketch, and 

 will be given hereafter. As the diatomacea; live, grow, and multiply, 

 thus floating freely on the surface, along the bottom, or through the 

 mass of the water, or wave in tiny filaments from other objects, they 

 must die; and the most perishable part of their bodies, namely, the cell- 

 contents, will be dissolved in the water or dissipated in gases, to return 

 and again build up new individuals at some future time. But their less 

 perishable portions, their siliceous skeletons, will fall to the bottom of 

 the pond, lake, ocean, or river, and there collect. Their remains will 

 also be found in the stomachs of such animals as are vegetable feeders, 

 as are most of the mollusca, like the oyster, the clam, and the water 

 snails, as well as the Crustacea,— lobsters, crabs, and the like. So, like- 



