54 PRACTICAL BOTANY. 



IV. In order to study the process of thickening of 

 the midrib, cut transverse sections successively of older 

 (i.e. lower) parts of the stalk: treat as before, and 

 compare them. It will then be recognised 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 ultimately 

 thrown off; the inner cortical tissue however remains 

 active, the cells increase in size, dividing periclinally, 

 and form a massive band, easily recognised by the 

 naked eye. The medulla also increases greatly in bulk, 

 many new hyphal filaments being formed, while they 

 differentiate into two series : (a) smaller ones, with 

 sparing protoplasm ; (5) others of larger size, with a 

 granular protoplasmic lining. 



Longitudinal sections made at different points down 

 to the thickened stalk itself, will lend solidity to these 

 observations: in such sections note, especially in the 

 oldest parts of the stem, the different types of hyphae ; 

 also the origin of fresh hyphal filaments from cells of the 

 cortical band, as tubes which push their way through 

 the mucilaginous cell-walls of the earlier-formed tissue. 

 Look for endings of some of the filaments : note also the 

 circular pits in their lateral walls. 



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 acetic 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 sub- 

 stratum, hence the firmness of its hold. Foreign bodies 



