184 STRUCTURAL BOTANY 



range than the Chlorophycese, for though no brown Algse 

 are quite so small or so simple as Pleurococcus, yet many 

 of them very much exceed any of the former division 

 both in size and complexity. The peculiar colour of 

 their thallus results from the fact that in addition to 

 the chlorophyll which they all contain, another pigment 

 of a brown colour is present, which more or less com- 

 pletely disguises the green of the chlorophyll. Unlike 

 the latter, the brown colouring matter is soluble in 

 fresh water, so that we can easily extract it and make 

 the chlorophyll visible. 



The Phseophyceae certainly form a natural group, 

 for from the lowest to the highest there are certain 

 points in their organisation which are common to all. 

 The colour in itself is not a character of much im- 

 portance, but it coincides roughly with structural 

 features, and affords a useful external mark by which 

 the group can in most cases be recognised. This 

 mark must, however, be used with caution, for there 

 are some Algse which resemble the Phseophycese in 

 colour, but have otherwise nothing in common with 

 them. 



The majority of the Phseophycese are reproduced by 

 zoospores ; these are called the Phceo-zoosporece. A second 

 order only forms sexually produced resting - spores ; 

 this is the family Fucacece. We will take one example 

 of each, for space will not allow us to do more, though 

 really a large number of types would be necessary if 

 our object were to gain any adequate idea of the diver- 

 sity of the brown seaweeds. 



