222 ELEMENTS OF BOTANY 
of plants which together make the great group called 
Alge. Something of its importance in nature is indi- 
cated by the following facts. The number of known 
species is about 12,000. In size, the individuals in vari- 
ous species range from a single cell of microscopic dimen- 
sions, as in Plewrococcus, to the giant kelp of California 
which reaches a length of more than 1000 feet. The 
form ranges from a simple spherical cell as in Plewro- 
coceus to an extensive, branching cell in other groups, 
and to specialized organs in the form of root, stem, leaf, 
air-bladder, and fruiting organs in Sargassum, which is 
an ally of Fucus. 
The alge illustrate a series of modes of propagation 
from simple division to the union of two similar masses of 
protoplasm to form a spore in Spirogyra and the direct 
fertilization of a germ-cell by motile antherozoids in Fucus. 
The alge fall into five natural groups based primarily 
on the mode of fruiting. 
THE STUDY OF BLACK MOLD 
(RHIZOPUS NIGRICANS) 
284. Occurrence. — This mold may be found in abundance on 
decaying fruits, such as tomatoes, apples, peaches, grapes, and cher- 
ries, or on decaying sweet potatoes or squashes. For class study it 
may mnost conveniently be obtained by putting pieces of wet bread 
on plates for a few days under bell-jars-and leaving in a warm place 
until patches of the mold begin to appear. 
285. Examination with the Magnifying Glass. —Study some of 
the larger and more mature patches and some of the smaller ones. 
Note: 
(a) The slender, thread-like network with which the surface of 
the bread is covered. The threads are known as hypha, the entire 
network is called the mycelium. 
