THE DIATOMACE.E OF PHILADELPHIA AND VICINITY 11 



method of rejuvenescence. When, however, the auxospores are thrown off from filamentous 

 diatoms, it is probable that two may conjugate, their contents dividing each into two daugh- 

 ter cells which unite into two zygospores. The usual method is the union of two frustules, 

 which, throwing off the old valves, coalesce into a single mass of protoplasm which produces 

 an auxospore, sometimes called a sporangial frustule. It is stated that in some cases two 

 frustules coalesce and produce two auxospores. 



The existence of spores in diatoms is a much-disputed point. While they have never 

 been seen, the inference that they exist is very great, as otherwise it becomes difficult to 

 understand the sudden growth of species in localities and under conditions that seem to 

 preclude the actual presence of the living frustule. It is a matter of common observation 

 that, in examining collections of living forms, minute frustules or brownish globules appear 

 to resemble larger diatoms. In gatherings of Gomphonema, when many specimens are 

 sessile on the same object, numerous intermediate sizes, varying from minute globules to 

 the type, are seen, yet not positively demonstrable as the same. 



Conjugation, the formation of auxospores, and the actual process of cell division are 

 seldom seen, as they occur during the night or at least in darkness. It is advisable in order 

 to observe reduplication to obtain the material about midnight and place it in very dilute 

 alcohol. In filamentous forms, however, the cell division is easily observed at any time in 

 its various stages. By immersing in picric acid (saturated solution), transferring to very 

 dilute alcohol which is gradually increased in strength, and then passing through oil of 

 cloves and finally to the mounting medium, excellent preparations can be made. By stain- 

 ing with gold chloride alone the nucleus is made apparent without further treatment. 



EVOLUTION OF FORMS 



It may be assumed that diatoms originated in the sea; to deny this requires evidence 

 of the existence of fresh-water species previous to the Miocene period which is entirely 

 marine. In those subject to fluctuations of the waves, as pelagic diatoms, their existence 

 appears to be contingent upon the methods by which the separate frustules can cohere. 

 Various devices, including hooks, spiral bundles, horns and processes exuding threads of 

 plasma, exist for holding together the frustules. When marine forms are found in quiet 

 waters some of these devices, being no longer of any value, cease to grow, although free 

 swimming diatoms are rare. They either occur in long chains or are stipitate or sessile. If 

 it is further assumed that the fresh-water diatoms are found in greater abundance in later 

 periods, the action of running streams makes necessary the provision of some means by 

 which the species may continue to colonize. This may be recognized in the occurrence of 

 linear forms chiefly in streams. Circular forms, such as Cyclotella which have no raphe, 

 are found in quiet waters, such as pools or ditches, and never exist living in running streams. 

 Those forms only would be able to live in water having a more or less swift current under 

 one of three conditions: they must, as in Gomphonema, be adherent to surrounding objects 

 by a stipe; or be enclosed in a gelatinous tube, as in Homceocladia; or have an independent 

 motion powerful enough to overcome the influence of the current. It is true that many forms 

 with a raphe have no apparent motion. In the case of Mastogloia provision is made in a gelat- 

 inous cushion in which the frustules are preserved. In Cocconeis, with a true raphe in one 

 valve only, in Epithemia, with a partial raphe, or in certain Eunotise with a trace of one, we 

 find species evidently degenerate and parasitic. The long Synedrae, having only a median 

 line, live in running streams, since they are attached at one end to other algae. Forms with 

 a true raphe appear to be more highly developed, since they are able to seek locations favor- 

 able to growth. Given, therefore, the structure of the valve, the habitat may be inferred. 



