250 STRUCTURE AND LIFE HISTORIES 
generations is not alike. The sporophyte, for example, 
begins life as a parasite on the gametophyte, while the 
gametophyte leads an independent existence from the 
start. The reader may also recall other differences. The 
question we have here raised, however, is a very funda- 
mental one, and will be further discussed in Chapter 
XXII. 
ULOTHRIX 
237. Habitat. The genus Ulothrix occurs everywhere, 
from pole to pole, in fresh water. 
238. Description. The plant body (Fig. 262) is usually 
a simple thread of cells, though in exceptional cases the 
cell-divisions result in a cell-plate instead of a thread. 1 All 
the cells are similar in appearance and structure, except 
the basal one, by which the plant is attached to some solid 
body. This cell is somewhat larger than the others, 
possesses less pigment, and is suitably modified to serve 
as an organ of attachment, or hold-fast. The protoplast 
of each cell possesses one nucleus, surrounded by the green, 
cylindrical chloroplast. No other pigment occurs, and 
therefore Ulothrix belongs to the Chlorophyceae, or green 
algae. Photosynthesis, of course, takes place, and a 
portion of the photosynthate may become transformed 
into starch, the presence of which is easily demonstrated 
by the usual test with iodine. 
239. Asexual Reproduction. Every cell of the plant, 
except the hold-fast, is capable of functioning as a re- 
productive cell, and two methods of reproduction are 
common. In one case the entire protoplast of a cell 
1 These two forms of plant bodies are sometimes designated by the 
terms "linear aggregate" (a filament) and "superficial aggregate" (a 
plate). 
