Ulvaceas 



277 



It is in the genus Ulva that the thallus is most securely attached. The attachment is 

 by a basal disc, which is firmly adherent to stones, wood, or other Algse, or as pointed out 

 by Cotton ('11), it is held down in muddy estuaries by the byssus threads of mussels. 

 The attaching disc was first described by Thuret ('78), and has been recently investigated 

 by Delf ('12). The disc is composed of interwoven 'disc-filaments' which are tubular 

 prolongations of the thallus-cells. The prolongations may be given oft' by any of the more 

 basal cells of either of the layers composing the thallus ; they pass down between the two 

 layers of cells, pursuing a rather intricate and sinuous course in the lower part, and they 

 sometimes branch. Thuret states that the filaments may attain a length of 6 10 mm., 

 but Delf never fcmnd them to exceed 3 mm. At the periphery of the disc the extremities 

 of the filaments swell out, become coherent and multinucleate, several transverse walls 



Fig. 178. Letterstedtia insignis Aresch. 1 nat. size. 



often arising, so that a pseudoparenchyma is produced. The thallus-cells which give rise 

 to the filaments appear to be always multinucleate, from three to five nuclei having been 

 observed in their upper part ; the minute nuclei also occur at intervals in the filaments 

 and several are present in their swollen tips (Delf, '12). 



In cases where Ulva is attached to certain of the smaller red seaweeds, Delf has found 

 that the disc often completely encircles the host-plant (consult fig. 177 G) and that the 

 filaments may penetrate in a parasitic manner into the cells of the host. 



Propagation occurs in some species of Monostroma and Enteromorpha by 

 gemmation or proliferation of the thallus, the small portions which are thus 



