INTRODUCTION. 



The following preliminary remarks may prove interesting to 

 those who have not heretofore paid much attention to the sub- 

 ject which has inspired the preparation of this volume. 



The term Algw signifies Sea-weeds^ and is used to designate 

 certain marine and fresh- water plants, which because they bear 

 no flowers, stamens nor pistils, and in fructification produce 

 spores instead of seeds, are styled cryptogamous plants. The 

 Algce comprise not only sea-weeds properly so-called, but like- 

 wise the gelatinous, or scum-like substances found floating on or 

 near the surface of ponds, ditch water and placid streams ; only 

 a very small proportion of the entire class of fresh-water Algce 

 is confined to trunks of trees, shady recesses, or to rocks dripping 

 with moisture. 



Owing to the life-like peculiarities exhibited in some sfages of 

 their development and growth many of the Algm were believed 

 by Ehrenberg and other micoscropists of his time, to belong to 

 the animal kingdom, but the wholly vegetable character of the 

 Algw is now too well established to admit of further controversy. 



Howsoever great in other respects their individual differences 

 may be, the Algm possess certain characteristics which are com- 

 mon to them all. They are cellular, flowerless and devoid of 

 roots ; their home is in the water ; the very few which atfect 

 other localities, die when deprived of moisture. Their nutri- 

 ment is absorbed through their entire surface from the medium 

 in which they live. They are totally devoid of vascular tissue, 

 in fact, are merely congeries of simple cells on the arrangement 

 of which depends their structural differences. 



Pew classes of plants present greater diversities of form than 

 do the Algw. Some are minute enough to tax the powers of our 

 best microscopes, while others are a constant source of astonish- 

 ment because of their enormous size, stretching as some of them 



xi 



