212 THE MICKOSCOPE. 



half- tide level; and is rarely seen in those which grow at 

 any great depth. But to this rule there are exceptions. 



" Several of the more perfect Confervoidece and Sipho- 

 nacece grow beyond the reach of ordinary tides; and others 

 are sometimes dislodged from considerable depths) as the 

 Anadyomene. 



" The olivaceous-broim, or olive-green, is almost entirely 

 confined to marine species, and is, in the main, charac- 

 teristic of those that grow in half- tide level, becoming less 

 frequent towards low-water mark ; but it frequently occurs 

 also at greater depths, in which case it is very dark, and 

 passes into brown, or almost black. The red, also, is 

 almost exclusively marine, and reaches its maximum in 

 deep water. When it occurs above half- tide level it 

 assumes either purple, or orange, or yellow tints, and 

 sometimes even a cast of green; but in these cases it is 

 sometimes brightened by placing the specimen for a short 

 time in fresh water. It is rarely very pure within the 

 range of extreme low-water mark, higher than which 

 many of the more delicate species will not vegetate; and 

 those that do exist, degenerate in form as well as in 

 colour. 



" How far below low- water mark the red species extend, 

 has not been ascertained, but those taken from the 

 extreme depths of the sea, are, like the olive series, found 

 with the darkest and richest colouring. 



"The green species are of the simplest structure, and 

 differ remarkably in their mode of propagation from either 

 of the other tribes, their seeds being endowed at the period 

 of germination with a rotatory motion. The olivaceous 

 are the most perfectly compound, and reach the largest 

 size ; and the red form a group distinguished not less by 

 their beauty and delicacy of their tissues, than by producing 

 seeds under two forms, thus possessing what is called a 

 double fructification." 



The student must not entirely depend upon colour in 

 referring plants which he may gather to their places in a 

 system. Laurencia pinnatifida, growing near low- water 

 mark, is of a fine, deep, purple-red a little higher up it 

 is a dull purple-brown ; higher up still, a pale brownish- 

 red, and at last, near high-water-mark, it is often yellowish 



