278 MICROSCOPIC FORMS OF VEGETABLE LIFE. 



interior the aggregation of separate organs which have been 

 imagined to exist in it. 



173. The Diatomacese seem to have received their name from 

 the readiness with which those forms that grow in coherent 

 masses (which were those with which Naturalists first became 

 acquainted) may be cut or broken through ; hence they have been 

 also designated by the vernacular term "brittle-worts." Of this 

 we have an example in the common Diatoma (Fig. 94), whose 

 component cells (which in this tribe are usually designated as 

 frustules) are sometimes found adherent side by side (as at b) so 

 as to form filaments, but are more commonly met with in a state 

 of partial separation, remaining connected at their angles only 

 (usually the alternate angles of the contiguous frustules) so as to 

 form a zigzag chain. A similar cohesion at the angles is seen 

 in the allied genus G-rammatophora (Fig. 95), in Isthmia (Fig. 96), 

 and in many other Diatoms; in Biddulphia (Fig. 81), there even 

 seems to be a special organ of attachment at these points. In 

 some Diatoms, however, the cells produced by successive acts of 

 binary subdivision, habitually remain adherent one to another; 

 and thus are produced filaments or clusters of various shapes. 

 Thus it is obvious that, when each cell is a short cylinder, an 

 aggregation of such cylinders, end to end, must form a rounded 

 filament, as in Meloseira (Figs. 97, 98) ; and whatever may be the 

 form of the sides of the cells, if they be parallel one to the other, 

 a straight filament will still be produced, as in Achnanthes (Fig. 

 93). But if, instead of being parallel, the sides be somewhat 

 inclined towards each other, a curved band will be the result; 

 this may not continue entire, but may so divide itself as to form 

 fan-shaped expansions, as those of Lichmophora flabellata (Fig. 

 91) ; or the cohesion may be sufficient to occasion the band to 

 wind itself (as it were) round a central axis, and thus, not merely 

 to form a complete circle, but a spiral of several turns, as in Me- 

 ridion circulars (Fig. 92, B). Many Diatoms, again, possess a 

 stipes or stalk-like appendage, by which they are attached to 

 other plants or to stones, pieces of wood, &c., and this may be a 

 simple foot-like appendage, as in Achnanthes longipes (Fig. 93), 

 or it may be a composite plant-like structure, as in Lichmophora 

 (Fig. 91), Gomphonema (Fig. 89), and Mastogloia (Fig. 99). Little 

 is known respecting the nature of this stipes; it is, however, 

 quite flexible ; and may be conceived to be an extension of the 

 cellulose coat unconsolidated by silex, analogous to the prolonga- 

 tions which have been seen in the Desmidiacece ( 163), and to the 

 filaments which sometimes connect the cells of the Palmellacece 

 (, 194). Some Diatoms, again, have a mucous or gelatinous in- 

 vestment, which may even be so substantial that they lie as it 

 were in a bed of it, as in Mastogloia (Figs. 99, 100), or which 

 may form a sort of tubular sheath, as in Schizonema. In a large 

 proportion of the group, however, the frustules are always met 

 with entirely free ; neither remaining in the least degree coherent 



