94 HYDKODICTYEtE. 



Order YIII.— HYDRODICTYE^. 



Hydrodictye^, Kiitz. Phyc. Gen. p. 281. Sp. Alg. p. 448. Berk. Crypt. Bot. p. 

 138. Dne. Class, p. 31. (in part only.) 



Diagnosis. Green (fresh-water) Alg^, composed of cylindrical cells, united by their 

 ends into a saccate net-work, with polygonal meshes ; each side of the mesh formed of 

 a single cell. Endochrome of each cell resolved at maturity into indefinitely numerous, 

 minute zoospores., which arrange themselves, end to end, into a new net-work, whilst 

 still contained within the parent cell. Kets viviparous. 



Natural Character. The genus Hydrodictyon differs so remarkably in the mode 

 of evolution of its frond from that of any other confervoid Alga that it has been found 

 necessary to constitute it the type of a distinct family. Its essentially distinctive 

 characters are thus well given by Messrs. Derbes and Soliere in their able memoir : 

 " Each zoospore of this plant gives birth to one cell only, whose further development 

 will consist merely in an increase of dimensions, without undergoing any multiplication. 

 Here then, without doubt, is the most distinctive character of the genus ; for in the 

 Conferva;., Avith which it has the greatest affinity, one zoospore gives birth to an 

 individual, which increases in dimensions by the multiplication of its cells ; here, on the 

 contrary, a great number of zoospores unite together to form an individual, which is 

 composed of a limited number of cells, which number remains the same during the whole 

 duration of the plant ; that is to say, until each of these cellules, in its turn, gives birth 

 to a young Hydrodictyon complete. In other terms, a Hydrodictyon is an assemblage 

 of little plants reduced to a single cell, formed by the development of a zoospore." If 

 we trace the development, it will be obvious that this is a true explanation of the 

 viviparous net-work. 



At all stages of its growth, then, the structure of the Hydrodictyon is the same. 

 Young specimens differ from old ones merely in the size of tlie cells of which the net is 

 composed ; the number of the cells, their form, and that of the net are the same in 

 young as in old nets. In all stages the Hydrodictyon is a bag-like or purse-shaped 

 net, with polygonal, generally five-sided meshes, each mesh consisting of a single 

 articidation or cylindrical cell, united by its ends to the neighbouring cells, just as the 

 cells of a Conferva are united, but liaving no passage from cell to cell, and each cell, 

 from first to last, carrying on an independent existence. When first emitted from the 



