SEA WEEDS 351 



to consist of a single layer of cells only, while in the larger forms 

 the Ulvacece, for example the thallus may be formed of two or three 

 distinct layers, and some of the cells may be elongated into tubes. 



A remarkable feature of the green- spored weeds is the large 

 size of the chlorophyll granules as compared with those of the 

 other groups, and also the great variety of forms which these 

 granules assume. They may be easily seen under a low power, 

 and the examination of the weeds will show that the thalli are not 

 uniformly green, but that the colour of the plants is due entirely 

 to the chlorophyll granules, the remainder of the plant substance 

 being quite colourless. 



If a green sea weed be placed in alcohol for a short time, it will 

 be found that the liquid assumes a green colour, while the plant 

 itself becomes colourless. The explanation is, of course, that 

 chlorophyll is soluble in alcohol. The presence of starch also in 

 the weed may be proved in a very simple manner, as follows : 

 Mount a small piece in water, and then put a drop of iodine solution 

 by the edge of the cover-glass. The solution will gradually diffuse 

 itself around the object, turning the starch-grains to a deep blue 

 colour, and so rendering them very conspicuous under a moderately 

 high power. 



The manner in which the green weeds are reproduced is very 

 interesting also. In some cases the fragments of a thallus that 

 have been detached by storms or other mechanical means possess 

 the power of independent growth, and develop into plants ; and this 

 mode of reproduction may often be watched in the indoor aquarium. 

 Another method is by the agency of little spores (zoospores) that 

 are produced at the edges or extremities of the thallus. Certain 

 of the cells become modified into what are called zoosi>orangia, 

 and the minute zoospores are formed within them. The walls of 

 the cells either gradually degenerate, or are fractured, and the 

 zoospores are thus set free. The latter are provided with little 

 vibratile cilia, by which they move about freely in the water. 

 Some eventually settle down and germinate without any further 

 aid, but others are unable to develop until they have been fertilised 

 by fusion with another cell. The former is therefore an asexual 

 development, while the latter is sexual. 



Some of the delicate, filamentous green algae are reproduced by 

 another process termed conjugation. In this case two adjacent 

 threads that lie close together become lightly united by a covering 

 of gelatinous substance, and a cell of each throws out a process. 



