5 88 MICROSCOPIC FORMS OF VEGETABLE LIFE THALLOPHYTES 



parts separate from each other. This is well seen in the genus 

 Diatoma, formed of rectangular individual frustules, where the 

 arrangement resulting from the principle of lateral union causes 

 them to develop into filaments or zigzag chains, the frustules remain- 

 ing perfectly distinct, and united only by a small isthmus or cushion 

 at the angles. A similar cohesion at the angles is seen in the allied 

 genus Grammatophora (fig. 452). in Isthmia (fig. 457), and in many 

 other diatoms ; in Biddulphia (fig. 445) there even seems to be a 

 special organ of attachment at these points. In some diatoms, 

 however, the frustules produced by successive acts of binary subdi- 

 vision habitually remain coherent one to another, and thus are pro- 

 duced filaments or clusters of various shapes. Thus it is obvious 

 that when each frustule is a short cylinder, an aggregation of such 

 cylinders, end to end. must form a rounded filament, as in Melosira 

 (fig. 444) ; and, whatever may lie the form of the sides of the 

 frustules, if they be parallel one to the other a atraight filament 

 will be produced, as in Achnanthes (fig. 461). 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 Licmophora flabellata (fig. 450) ; or the cohesion may be sufficient 

 to occasion the Viand to wind itself (as it were) round a central axis, 

 and thus to form, not merely a complete circle, but a spiral of several 

 turns, as in Meridian circtdare (fig. 448). Many diatoms, again, 

 possess a stipe, or stalk-like appendage, by which aggregations of 

 frustules are attached to other plants, or to stones, pieces of wood, 

 Arc. ; and this may be a simple foot-like appendage, as in Ai-limiiitJifx 

 longipes (fig. 461), or it may be a composite plant-like structure, as 

 in Licmophora (fig. 450). (lompJtonema, (fig. 462), and Mnstoylo'm 

 (fig. 465). Little is known respecting the nature of this stipe ; it- 

 is, however, quite flexible, and may be conceived to be an extension 

 of the cellulose coat, unconsolidated by silex, analogous to the 

 prolongations which have been seen in the Desmidiacew, and to the 

 filaments which sometimes connect the cells of the Palmettacece. 

 Some diatoms, again, have a mucous or gelatinous investment, which 

 may even be so substantial that their frustules lie as it were in a 

 bed of it, as in Jfastoyloid, (figs. 465 B. 466), or may form a sort of 

 tubular sheath to them, as in Schizonema (fig. 464). In a large 

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

 entirely free, neither remaining in the least degree coherent one to 

 another after the process of binary subdivision has once been com- 

 pleted, nor being in any way connected, either by a stipe, or by a 

 gelatinous investment. This is the case, for example, with Tricera- 

 fi/iui. (fig. 442), Pleurosigma (Plate I. figs. 1, 2). Actinocyclus, 

 Actinoptychus (tig. 467), Arachnoidiscus (Plate Xll), ('i>i]>i/l<idiscus 

 (lig. 4r*4J,,sW//v/A/ dig. 45:',), Coscinodiscus (Plate l,figs. :i. 4.'lig.455), 

 lliin,j>,'ltii , and many others. The solitary discoidal forms, however, 

 when obl.-iined in their living state, are commonly found cohering 

 1<> the surface of aquatic plants. 



\\ e have now to examine more minutely into the curious struc- 

 ture of the silicitied casing which encloses every diatom-cell or 



s> 



