THE ALG.E p> 
from one cell to the other. Then the protoplasm from one 
cell passes through this tube to the other cell, and the two 
masses of protoplasm unite. The nuclei unite and one cell is 
formed from what were two. About this new cell a heavy 
wall is formed, and this specially formed structure is a spore ; 
that is, a specially formed cell which can reproduce the kind 
of plant that formed it. It must be noted also that this spore, 
being formed by cell union and not by cell division, is sexual ; 
also, sexual cells that are formed by union of similar cells are 
called sygospores, that ix, yoked spores. It is customary to 
speak of the cells that unite to form spores as gametes, and in 
the case of the plant Np/royyra the gametes are similar, so 
there need be no special name here for the two gametes, to dis- 
tinguish them one from another. In asexual reproduction, as 
will be seen in other plants, reproduction is carried on by means 
of spores that are formed by cell division, not by cell union. 
In two Spireyyra plants there may be many cells uniting, 
or conjugating, at the same time. These pairs are usually in 
about the same stage of spore formation. Occasionally cells 
from one plant will unite with those from more than one other 
plant. Also, occasionally one cell may unite with the adjoin- 
ing one in the same plant. Whien ripe, the zygospores are set 
free by the decay of the old walls and may fall to the bottom 
of the pond or stream. After a period of rest, sometimes after 
a drought or in the spring, these spores germinate and pro- 
duce new Spirogyra plants. It is obviously an advantage to 
the plant to have a heavy-walled spore to carry it through 
unfavorable periods. 
211. A branching alga: Cladophora. This is a green alga 
of very wide distribution. It usually grows attached to objects 
along shoals in streams, over dams, and about waterfalls. 
Sometimes it appears in heavy floating mats along margins of 
ponds, lakes, and even oceans. It is one of the few green 
alge ever found in salt water. Cladophora is extensively 
branched (fig. 176), and since its branched filaments are 
etandantly supplied with chlorophyll, it is clear that this 
