454 PHYSICAL GEOGRAPHY. 



develop around themselves siliceous shells or skeletons, and become dia- 

 toms of the normal dimensions of the species from and by which the 

 sporangium was produced. On the other hand, the sporangia may con- 

 stitute the "resting" state, such as is known to occur in several simple 

 forms of life, in which the species encounters the severity of the winter 

 only to reproduce the species in the spring. In this case, examination of 

 localities, known to produce certain species during the summer, should, 

 during the winter months, be searched when most likely there would be 

 found abundance of sporangial forms. I am not aware that any such 

 investigations have been as yet made; and the difficulties surrounding 

 the study of these organisms is so great that but few have the patience 

 requisite for such work. Hence we find that most of the papers relating 

 to the diatomaceas, which have been published, are by persons who de- 

 light in the naming of "new species," and have not cared to spend the 

 time necessary to determine whether they be but transition forms, spo- 

 rangia, or true species. 



Although, then, as has been said, the whole life-history of the diato- 

 maceous sporangium has not been established, yet we know enough to 

 convince us, as Prof. Smith says, that "the ordinary diatomaceous frus- 

 tule seems to owe its production to the protoplasmic contents of the 

 sporangial frustule formed by the process of conjugation. These spo- 

 rangia, like the seeds of higher plants, often remain for a long period 

 dormant, and are borne about by currents, or become imbedded in the 

 mud of the waters in which they have been produced, until the circum- 

 stances necessary to their development concur to call them into activity. 

 At such times, their siliceous epiderms open to permit the escape of 

 the contained endochrome, which is resolved into a myriad of embryonic 

 frustules ; these either remain free, or surround themselves with mucus, 

 forming a pellicle or stratum, and, in a definite but unascertained period, 

 reach the mature form of the ordinary frustule." Prof. Smith has made 

 observations which appear to establish this fact of the formation of 

 motile spores, which he details in the following words : " In the gathering 

 of Cocconema Cistula, made in April, 1852, which contained numerous 

 mstances of the conjugating process, I observed the frequent occurrence 

 of cysts enclosing minute bodies, variable in their number and size, and 

 many of which had the outline and markings of the surrounding forms. 



