FUCUS. 55 



may often be seen embedded in this part of the thallus, 

 and this finds its explanation in the fact that the tissue 

 here consists of hyphse similar to those of the medulla, 

 and each appears to grow in an independent manner. 

 Examine the section under a high power, and it will be 

 seen that the mass of tissue resolves itself at the surface 

 of attachment into a number of separate filaments, 

 each of which applies itself separately to the surface of 

 the substratum. Compare drawings or preparations of 

 young plants (see below, page 62). 



VI. Cut thin longitudinal sections through the apex 

 in a plane perpendicular to that of the flattened thallus : 

 treat those sections which are median with glacial 

 acetic acid, and mount in a mixture in equal parts of 

 pure glycerine and glacial acetic acid : examine under 

 a medium power, and observe 



1. The outline of the section, showing a depression of 

 the apex corresponding to the groove already recognised 

 with the naked eye. 



2. That from their arrangement it may be concluded 

 that the various tissues of the thallus are derived from 

 an initial point at the base of the depression. 



3. Trace the cuticle as continuous from the free outer 

 surface into the slightly enlarged cavity of the depres- 

 sion, which is itself filled with a mucilaginous secretion 

 lying between the cuticle and the cellulose walls of 

 the adjacent cells. 



4. Examining the tissue at the base of the depres- 

 sion, if the section be median, a single large cell 

 having the form of a truncated pyramid may be 

 recognised in a central position : this is one of the 

 initial cells. 



