THALLOPHYTA— ALG^ 



57 



Siphoned. — These are the most remarkable forms of the 

 group, the thallus consisting of a single coenocyte, often of very 

 large dimensions, or of a number of ccenocytes attached to each 

 other, forming a filament. The largest members of this family 

 belong to the genus Caulerpa, which in many cases simulates 

 the different types of habit exhibited by terrestrial plants. The 

 stem and leaves are nearly always well represented, and the root 

 system is clearly differentiated. The internal cavity of the 

 coenocyte is crossed by interlacing strands or trabeculse formed 

 of a modification of cellulose, and springing froni the outer wall 

 of the structure. A form of much humbler type, but more 

 familiar to us from its occurrence in fresh-water and in muddy 



Fie. 805. 



Fig. 805. A. s^. Newly formed zoospore or zoogouidium of Vaucheria sessilia 

 escaping. B. Zoospore at rest after having lost its cilia, c. First stage of 

 germination. D. Filament of Vaucheria sessilis producing oogouia, 0(j, off, 

 and antheridium, h. w. Hyaline root-like process, forming a sort of 

 mycelium, sq. Zoospore, which by germinating lias formed the filament. 

 After Sachs. 



places, is Vaucheria (fig. 805), The plants are filamentous 

 ccenocytes, which branch irregularly or dichotomously. There 

 is but little differentiation, but from some of the filaments curious 

 root-like structures are developed for purposes of attachment. 

 Other forms are Aceiabidaria, where there are stalks crowned 

 by whorls of coherent leaves, and Botrydmm {fig. 804), where a 

 bladder-like head is continued downwards by a slender stalk to 

 a much-branched root, aU being a single ccenocyte. Forms 

 composed of several ccenocytes united into a filamentous thallus 

 are shown by the genera Sphceroplea, Cladophora, &c. The 

 curious genus Hydrodiotyon is sometimes referred to this 

 group. 



