XVI INTRODUCTION. 



differ remarkably in their mode of propagation from either 

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

 of germination with a sort of motion, which some have 

 called voluntary, but which does not really possess that 

 animal property. The olivaceous are the most perfect and 

 compound in the structure of their organs of vegetation, 

 and reach the largest size ; and the red form a group dis- 

 tinguished not less by the beauty and delicacy of their 

 tissue, than by producing spores under two forms, thus pos- 

 sessing what is called a double fructification. Hence, 

 modern botanists, since the publication of Lamouroux's 

 system, have, whatever their particular views of arrange- 

 ment may be, almost invariably used colour as one of the 

 principal characters on which their systematic arrangement 

 is based; and to a great extent it may be safely trusted. 



But the young student must be careful not to place too 

 absolute dependance on colour alone, in referring plants 

 which he may gather to their place in the system ; for some 

 species, which in their healthy state are red, or of that 

 class of colour, become, when growing under unfavourable 

 circumstances, of an orange, yellowish, whitish, or greenish 

 shade. Laurencia pinnatijida is particularly variable in 

 this respect. When this species grows near low-water 

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

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

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

 greenish. The other species oi Laurencia vary in similar 

 but less striking degrees. Chondrus crispus too, when found 

 in shallow water, exposed to strong sunlight, is often of a 

 bright herbaceous green ; and Ceraniium ruhrum passes 

 through every shade of red and yellow, and at last degene- 

 rates into a dirty white, before it ceases to grow. All these 

 species vary in form and size, as they do in colour, and the 



