FTJCTJS. 279 



(1) The superficial cells, forming the limiting or epider- 

 moid layer, are in surface view rectangular or polygonal, 

 and arranged in longitudinal rows, but in T. S. and L. S. 

 of thallus are prismatic and vertically elongated. 



(2) Below this layer come wider cells, increasing in size 

 as we pass towards the interior of the thallus these cells, 

 with the epidermoid layer, form the cortex. 



(3) The internal tissue or medulla consists of elongated 

 cells joined end to end to form filaments which run parallel 

 to each other and to the long axis of the midrib and 

 are embedded, in mucilage. 



(4) The medulla of the midrib is continued at each side 

 into that of the wings, where the filaments form a loose 

 network. 



(5) The cells of the cortex and medulla have pitted walls, 

 the cross-walls in the medulla filaments resembling sieve- 

 tubes in appearance. 



(6) All the cells contain protoplasm and a nucleus, also 

 rounded chromatophores, which are abundant in the 

 outer cortex and scanty in the medulla. 



403. Structure of Air Bladder. In sections passing through 

 an air bladder of Fucus vesiculosus, note that the wall of the air- 

 filled cavity consists of a cortical tissue, with a lining of medullary 

 tissue which is loose and disorganised. Cut across a young bladder : 

 it contains a network of filaments like that seen in the wings, but 

 this is torn as the jelly is replaced by air during the growth and 

 expansion of the bladder. 



404. Growth in Thickness of Midrib. In T. S. and L. S. 



through successively older parts of the thallus, trace the processes 

 which lead to growth in thickness of the midrib, accompanied by 

 disappearance of the wings. As the thallus grows older, the outer 

 cells of the midrib cease to divide, and this tissue is thrown off. 

 The inner cortex remains active, increasing in bulk by division of 

 the cells, and also producing finger-like prolongations which grow 

 into the medulla, dividing by transverse walls and undergoing 

 branching ; these new filaments may be distinguished from the old 

 ones among which they are intruded by their smaller diameter and 

 lighter- coloured contents. 



405. Sterile Conceptacles. In sections that include one or 

 more of these structures, note the flask-like cavity, surrounded by 

 the inner cortex and opening on the surface of a conical projecting 



