33 



in th.e wings of the thallus ; scattered in tKe jelly 

 belonging to these threads are bubbles of gas, 

 which tear the loose tissue, and form the 

 chambers of the bladders. Old bladders are 

 hollow and filled with air and serve as swimming 

 bladders, the outside walls are thick, being 

 formed by successive divisions of the epiderm oid 

 layer. 



(6) The Stem. The tballus in its oldest part is 

 stem-like, and developed from the midrib, the 

 wings of which have gradually died off. The 

 innermost layer of the cortex (the thickening 

 layer) begins its activity at an early stage ; at a 

 little distance below the apex, tube-like prolong- 

 ations are developed, which grow downwards 

 into the jelly separating the threads ; these 

 divide by cross-walls and branch from time to 

 time, these thickening threads rarely pass into the 

 wings. In cross-section the two kinds of elements 

 can be distinguished : — 



(i) Scattered cells with brownish contents (primary 

 tissue.) 



(ii) Numerous closely-packed cells with narrow 

 cavities and greenish contents (secondary 

 tissue). 



While this has been going on the wings have died 

 off, the outer layer of the midrib has become brown 

 and dead and cast off, the second layer becomes 



