226 ULVAS. PART ii. 



of cells arranged in clusters containing some multiple 

 form of four, as in fig. 3, page 171. 



The frond membrane of the true Ulvas, as that of the 

 Ulva lactuca, is formed of but one layer of cells ; the 

 frond itself is thin as cambric paper, almost transpa- 

 rent, and of a pretty light green. When young it is a 

 puckered inflated bag, which afterwards bursts and 

 opens into a flat, ribless, wavy, more or less rounded ex- 

 pansion, three to six inches long, and as many broad. This 

 plant, which is attached to the rocks between the tide 

 marks on our shores, is rare in the Mediterranean ; nor is 

 it so common in Britain as the Ulva latissima (fig. 22), 

 which is cosmopolite, and abundant everywhere. It is 

 found as a ribless irregular expansion of a full bright 

 green in deep water, and of a yellow apple green when in 

 shallow water, and exposed to the light. The base and 

 stem are very short, and the frond, which the microscope 

 shows to be formed of two layers of cell membrane, 

 spreads so rapidly into crisp wide-lobed foliations, that 

 the parts often overlap each other in stiff bulging folds. 

 It is from six inches to a foot in height, and from three 

 to twelve inches wide. The frond of the Ulva Linza is 

 also formed of two layers of cells, but so small and so 

 closely pressed together that the two layers can only 

 be detected by the microscope. This plant, which is 

 from six inches to two feet long, is a ribless, narrow, 

 ribbon-shaped expansion with curled wavy edges taper- 

 ing to a base, and either blunt or pointed at the top. Its 

 colour is the same as that of the Ulva lactuca. 



In the Ulvas, which are multilocular plants, some cells 

 are selected to bear fruit, and others not. The granular 

 endochrome of these chosen cells divides into several 

 parts, which are at first in close contact and at rest ; 

 then they become restless, acquire four or a greater 

 number of cilia, and pass through a fracture in the cell 

 wall into the water, in which they swim freely as zoo- 



