INTRODUCTION TO CETPTOGAMIC BOTANY. 



203 



moreover, remarkable for tte beautiful reticulation of their 

 fronds, caused by the large hexagonal cells. The species are 

 extremely impatient of fresh water, in which they soon fade. 

 Bigenea is so infested with corallines, as for the most part to 

 present very shabby-looking specimens, and to appear as if it 

 appropriated to itself calcareous matter, which, however, is not 

 the case. The various species of Polysiphonia afford beautiful 

 objects for the microscope, transverse sections of their stems 

 exhibiting a great variety of regular, almost geometrical, figures, 

 from the mode in which the component tubes are arranged. 

 Some of the Bostrychice agree with them in this structure, and 

 in fact are scarcely distinguishable. 



181. Amongst the curious forms assumed by Algae, few are 

 more remarkable for beauty than those which present on the 

 one hand a clathroid, cancellated, or cribrose frond, or on the 

 other which exhibit an open net-work. Claudea is a beautiful 

 example of the cancellated frond, to which Heringia, a part 

 of which only is cancellated, will bear comparison, both for 

 beauty and elegance. Iridwa clathrata, Den., Ulva myrio- 

 trema, Crouan, and Agarum Gmelini, afford examples of 



Fig. 51. 



Portion of the net-work of Dictyurus purpurasoens, Bory., magnified. 

 From a specimen collected by Mr. Darwin. 



cribrose fronds; but there are others, like Hydrodictyon, which 

 exhibit the most elegant net-work. Hanovia, Rhodoplexia, 



