ulvace^:. 257 



specimens preserve their colour well, and adhere closely 

 to paper; but they are very susceptible of injury from 

 exposure to the air, or to a sudden change of tempera- 

 ture. The greatest care must, therefore, be used both 

 during the process of preparation for the herbarium, and 

 subsequently, whenever they may be examined, or they 

 will crack and curl up from the paper and be spoiled. 

 These remarks apply with equal, or greater, force to 

 dried specimens of all the species of this genus. 



Ulva latissima. The very broad Ulva. 



Frond composed of two layers of cellules, from a few inches 

 to two feet long, and about half as wide, extremely variable in 

 size and shape, with an irregular outline, and a ragged, waved 

 or sinuate margin, of very thin substance, and soft, but mo- 

 derately tough, when young of a vivid, somewhat bluish 

 green colour, becoming paler with age. Fructification dis- 

 persed throughout the frond. 



Ranging from between the tide-marks into ten or 

 twelve fathoms of water, and from the Equator to the 

 Arctic and Antarctic circles, this plant appears to be every- 

 where abundant, and to flourish almost equally in warm 

 and cold climates. It is to be found in summer and 

 autumn, and is annual. The fronds grow in tufts, and 

 are always flat ; they do not adhere to paper. Mature 

 specimens of this plant are frequently more or less 

 covered with minute, dot-like tufts of Myrionema. 



Ulva iactuca. The lettuce Ulva. 



Frond composed of a single membrane, or layer of cel- 

 lules, from two to six inches high, at first a pear-shaped sac, 



