174 SEAWEEDS 



ULVACE.E. 



General Characters. The Ulvacece are among the 

 first seaweeds that meet the collector's eye on reach- 

 ing the shore, since they grow for the most part at 

 high-water mark. The thallus is either a flat green 

 expanse of tissue, lobed irregularly, or hollow, 

 green, tubular and unequal. There are no special 

 sporangia, but the ordinary vegetative cells of the 

 thallus act as the parent cells of gametes, which 

 conjugate, and of zoospores with four cilia. 



The Thallus. In its most simple form, that of 

 Monostroma, the thallus consists of a flat layer of 

 cells, one cell thick, at least in the upper portion ; in 

 Ulva and Letterstedtia uniformly two cells thick, and 

 in Enteromwpha also two cells thick ; but these 

 layers soon separate, and produce a hollow space 

 between them, giving rise to the tubular forms that 

 are characteristic of the genus. The greater part of 

 the basal cells of the Ulvacece grow out into fila- 

 mentous processes which become irregularly inter- 

 woven, and, while serving as a fixing organ to the 

 substratum, add to the thickness of that part of the 

 thallus. 



While a certain degree of branching of the thallus 

 occurs in Enter omorpha, it is only in Letterstedtia 

 that lateral foliar appendages if they may be so 

 called of definite growth occur. These subse- 

 quently fall off from the older parts of the main 

 shoot, leaving it bare and irregularly toothed at the 

 margin. Pringsheimia is a very minute epiphyte 



