FUCUS SERRATUS 205 



the stalk : treat as before, and compare them. It will then be 

 recognized that, as the thallus grows older, the cells of the 

 limiting layer cease to divide by periclinal, and later also by 

 anticlinal walls : it becomes a quiescent tissue, and is ulti- 

 mately thrown off ; the inner cortical tissue however remains 

 active, the cells increase in size, dividing periclinally, and form 

 a massive band, easily recognized by the naked eye. The 

 medulla also increases greatly in bulk, many new hyphal fila- 

 ments being formed, while they differentiate into two series : (a) 

 smaller ones, with sparing protoplasm ; (b) others of larger size, 

 with a granular protoplasmic lining. 



V. Sections should be cut through the organ of attachment. 

 Take plants grown on wooden piles, or on limestone rock: in the 

 latter case the lime may be dissolved by dilute hydrochloric acid, 

 and the tissue then hardened in alcohol. Cut vertical sections, 

 and mount as before : note under a low power the irregularity 

 of the surface of attachment, which closely follows that of the 

 substratum, hence the firmness of its hold. Foreign bodies 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. 



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 recognized 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. Examining the tissue at the base of the depression, if the 

 section be median, a single large cell having the form of a 



