OF THE DIATOME^. 



75 



converts it into a sporangium. Beyond this stage, Mr. Thwaites does not 

 appear to have followed the sporangial fnistiile so generated ; but, assuming 

 the correctness of Prof. Smith's hypothesis of the generation and subsequent 

 evolution of numerous minute frustules mthin it, do we not find a precisely 

 analogous phenomenon with that which Eabenhorst represents as an addi- 

 tional mode of propagation, or with what Focke (see preceding page) describes 

 as the formation of gemmules out of the internal substance, and their sub- 

 sequent discharge? The supplementaiy phenomenon of alternation with 

 change of specific form, included in the statement of the latter observer, even 

 if confirmed, will not afi'ect the general analogy presumed. 



Habitats. — Appearance in masses, abundance, geographical distribution. 

 — Fossil Diatomece. — Existence in the atmosphere. — Practical uses and appli- 

 cations of the Diatomece. — The habitats and the distribution of the Diatomeae, 

 both in time and space, are the most extensive, various, and wide, of all 

 organic beings. In fresh, in salt, and in brackish waters they are ahke foimd ; 

 they exist abundantly in a h^ing state about the roots of plants and diffased 

 in moist earth ; they are also to be met ^vith in the dust of the atmosphere 

 and in meteoric products. They are, in fine, inhabitants of earth, air, and 

 water. When no longer ahve, their silicious skeletons preserve their form 

 and constant characters, iminjiu'ed by most of the causes which obhterate 

 the remains of other Hving beings. They are so. preserved in most of the 

 rocks above the oldest primaiy — in all, indeed, in which intense heat has not 

 operated to fuse sihca into a molten mass. At the present day they are 

 ejected from the bowels of the earth in the lava, cinders, and ashes of vol- 

 canos, and are borne about by the winds from one continent to another in 

 showers of dust. 



In respect of habitat, the Diatomeae are divisible into marine and fresh- 

 water species ; some indeed are common to both fresh and salt water, or 

 exist in brackish water. The following accoimt of the habitats of Diatomeae, 

 illustrated by reference to particular examples, is from the experienced pen 

 of Mr. Ralfs, who has supplied us with it : — 



" The Diatomeae may be obtained at all seasons of the year, but are most 

 plentiful in spring and summer, many of them indeed being hmited to that 

 period ; thus the species of Micromega and Schizonema are, with few excep- 

 tions, in perfection only in May and June, when they are met with in shel- 

 tered situations, forming wide patches on the ground and on the flat surfaces 

 of rocks exposed at ebb-tide. About the end of May the Enteromorpha 

 compressa, so common on our shores, often seems as if faded at the end ; this 

 appearance is frequently accompanied by the presence of Grammonema Jur- 

 gensii, which is easily recognized by its slippery feel, when from its pale 

 colour it would otherwise escape detection. 



'' At all seasons of the year, the smaller and more slender Algae, marine 

 and. freshwater, as soon as they attain maturity, become almost invariably 

 covered ^vith parasitic Diatomeae, which impart to them a brownish colour. 

 In this way we obtain species of Cocconeis, Achnanthes, Striatella, Tahellaria, 

 Grammatophora, Isthmia, Gornphonema, Podosphenia, Rhipidophora , and 

 Si/nedra. On the contrary, Amphitetras and Biddulphia prefer the muddy 

 cre\ices in the sheltered sides of pei-pendicular rocks. 



*' In salt marshes we may expect to find the Achnanthes subsessilis on the 

 slender filaments of Etiteromorpha, but so sparingly as hardly to discolour 

 them. The species of Epithemia are parasitic on Claclophora, both in brackish 

 and in freshwater pools. The Melosirce are common in marshes, especially at 

 the mouths of large rivers, where they form Conferva-hke brownish masses. 

 " Many of the unattached Diatomeae are produced in dark brown patches 



