256 FOUNDATIONS OF BOTANY 
plants which we have just studied, are types of several 
families of plants which together make the great group 
called Alge. Something of its importance in nature is 
indicated by these facts: The number of known species is 
about 12,000. In size, the individuals in various species 
range from a single cell of microscopic dimensions, as in 
Pleurococcus, 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 Pleuwrococcus to an extensive, 
branching cell in Vaucheria and its allies, specialized 
organs in the form of root, stem, leaf, air-bladder, and 
fruiting organs in Sargassum, which is an ally of Fucus. 
The algze illustrate a series of modes of propagation 
from simple division in Oscillatoria to the union of. two 
similar masses of protoplasm to form a spore in Spirogyra, 
the direct fertilization of a germ-cell by motile anthero- 
zoids in Vaucheria, Nitella, Fucus, the indirect fertilization 
of fruiting cells by non-motile antherozoids in Nemalion. 
In allies of the latter there are more intricate variations of 
the same mode. 
The alge fall into five natural groups based primarily 
on the mode of fruiting. In most cases color is coérdinate 
with class and may be relied upon as a superficial guide in 
grouping ; but there are a few exceptions, e.g., some fruit- 
ing like the red group are, nevertheless, green. 
The nutrition of the brown and the red alge is similar 
to that of the green algze, since the brown or red color 
merely conceals the green of the chlorophyll which is 
present in all and enables them all to take in and decom- 
pose carbon dioxide.! 
1See Murray’s Introduction to the Study of Seaweeds, pp. 4-6. London, 
1895. 
