378 



Conjugate 



(Turp.) Ehrenb. (W. & G. S. W., '98) and Staurastrum inconspicuum Nordst. 



(Borgesen, '01). Certain of the tropical species of Pleurotienium form longer 



or shorter filaments, and Micrasterias 

 foliacea Bail, is a true filamentous form of 

 a typically unicellular genus with complex 

 cell-outlines (fig. 239 B). 



The general evolution of the Placoderm 



('-! IWifi'l "^"--f 1 Desmids is fairly clear (vide G. S. W., '04), but 



<N /IPW S^'"-^ khe enus Roya has to be removed from the 



3--"rt sub-family Placodermas. Liitkemuller ('10) has 

 recently shown that the species of this genus are 

 Saccoderm Desmids and that Roya is a close ally 

 of Mesotxnium. 



It is highly probable that the complexity of 

 outline of the Desmid, which is so frequently 

 accompanied by a defensive armour of spines and 

 spinous processes, has been acquired as a means 

 of defence against the attacks of small aquatic 

 animals. After the loss of the filamentous con- 

 dition it became necessary for the solitary and 

 unprotected individuals to acquire some other 

 means of defence, and presumably the present 

 morphological complexity is the result. It is a 

 notable fact that those species which occur on wet 

 rocks and in other situations in which Amoebae, 

 Oligochsetes, Tardigrades and Crustacea are either 

 absent or very scanty, especially at high elevations, 

 usually possess a comparatively simple outline ; 

 D whereas those species occurring in deep bog-pools, 

 in the plankton, and at the quiet margins of deep 

 lakes, in which localities such enemies abound, are 

 generally possessed of a more complicated, and 

 in many cases of a formidable, exterior. These 

 characters acquired by the unicell are not only 

 protective against the depredations of aquatic 

 animals, but are also useful as anchors in the time 

 of floods. 



There are several thousand known 

 species of Desmids, almost all of which 

 can be readily identified by their external 

 morphological features. The majority of 

 Desmids have three principal axes of 

 symmetry at right angles to one another, 



and for this reason it is usually necessary to examine them in three positions. 



The most important aspect is the front view, in which the plant is observed 



Fig. 238. A and B, Gcnicularia elegans 

 W. & G. S. West, x433. C, Gona- 

 tozygon monotsenium De Bary var. 

 pilosellum Nordst., x 416. D, G. 

 aculeatum Hastings, x 416. 



