SECT. ii. ALG^E. 179 



SECTION H. 



ALGLE. 



THE principal objects in the study of plant-life are the 

 organs by means of which they obtain and assimilate 

 substances that are essential for their nourishment and 

 growth, and those by which the perpetuity of their race 

 is maintained and their type transmitted from age to 

 age. In the lowest group of plants, represented by the 

 Algae, which come first into consideration, the two pro- 

 perties are combined ; in the highest they are distinctly 

 different, but the progress from one to the other may 

 be traced through an ascending series of vegetable 

 structure. In the simple grades of vegetables, the pri- 

 mordial cell frequently constitutes the whole plant ; it 

 appears first, and then envelopes itself with a coat either 

 of cellulose or of a gelatinous substance. 



Many instances of this are to be found amongst 

 the Algae, which are all aquatic plants, and are found 

 growing either attached to other bodies, or floating in 

 dependently, and live, some species in fresh water, and 

 others in the sea and its estuaries. The Algae absorb 

 carbonic acid and give out oxygen, under the influ- 

 ence of sun-light, exactly as do the flowering plants ; 

 and the quantity of oxygen disengaged by them is said 

 to be enormous. 



Before proceeding to trace the structure and de- 

 velopment of the Algae, it may be desirable to indicate 

 something of the classification of this curious group of 



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