256 FOUNDATIONS OF BOTANY 



plants which we have just studied, are types of several 

 families of plants which together make the great group 

 called AlgcjR. 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 Pleurococcus 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 algae 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 algse fall into five natural groups based primarily 

 on the mode of fruiting. In most cases color is coordinate 

 with class arid 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 algae is similar 

 to that of the green algse, 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. 1 



1 See Murray's Introduction to the Study of Seaweeds, pp. 4-6. London, 

 1895. 



