278 



FUCT7S. 



401. Mucilaginous character of Thallus. Cut across 

 a fresh thalius, and note the slimy mucilage that oozes 

 out or is easily squeezed out. With a dry razor, cut thin 

 sections across a piece of thallus, held in pith; the sections 

 become twisted, since the outer tissues expand and the 

 inner tissues contract. Evidently in the intact thallus the 

 two tissues are in unequal conditions of tension, the outer 

 being compressed by the inner and the inner stretched by 

 the outer. Mount some sections in tap-water ; they swell 

 up greatly and become more distorted. Mount some in 

 sea- water ; the swelling is much less marked, hence fresh 

 material should be examined in sea- water. 



402. Structure of Thallus (Fig. 58). From the 

 upper region of the thallus, where the wings are well 

 developed, cut (a) horizontal sections, parallel to surface 



Fig. 58. Fucus. Part of a Transverse Section of the Thallus. See 402. 



of thallus, at different depths ; (b) transverse sections ; 

 (c) median longitudinal sections of midrib and of wing. 

 Mount the sections in glycerine, and note that 



