FUCUS SEKRATUS, L. (Wrack) 



OBSERVATIONS WITH THE NAKED EYE 



I. Of the various species of Fucus which are to be found on 

 our shores, the best adapted for laboratory work is Fucus 

 serratus : it is to be found near or below mid-tide level, and 

 may be distinguished from other species by its dark olive 

 colour, the flattened form and serrate margin of the branches of 

 the thallus, the absence of swollen " bladders," and the presence 

 of numerous dot-like conceptacles, crowded together on the 

 ends of branches which show no special swelling. 



Having recognized the species by these characters, examine 

 a well-developed plant with the naked eye, and note that the 

 thallus as a whole shows no differentiation of stem, leaf, and 

 root, as in vascular plants ; it consists of the following parts 



1. The flattened disk, of irregular outline, by means of which 

 the plant is firmly attached to the substratum. 



2. The stalk, which in old plants is of compressed cylindrical 

 form, but in young plants it may be clearly seen that it is 

 originally a broad flattened expansion, with a more or less 

 thickened midrib : a comparison of plants of successive ages 

 will demonstrate that the compressed cylindrical stalk results 

 from the thickening of the midrib, and decay of the lateral 

 wings. 



3. If this stalk be traced upwards it will be seen to branch 

 repeatedly, while on tracing the branches also upwards they 

 gradually assume the flattened form with serrate margin, thus 



