FUCUS VESICULOSUS. 243 



starch. The whole distribution of the tissues leads to the conclu- 

 sion that the processes of assimilation are localised in the cells of 

 the cortex, while the cells of the pith and the packing tissue furnish 

 the conducting system. If we treat the sections with chlorzinc 

 iodine, or with iodine and sulphuric acid, we obtain in either case, 

 but especially with the latter, a blue coloration of the walls. The 

 firmer membrane immediately surrounding the cell-cavity colours 

 deeply, the more distant less so ; in the loosest tissue, filling the 

 wings of the thallus, the staining of the jelly tends to be entirely 

 absent. The parts of the membrane around the cell-cavities show 

 distinct lamination. If we allow haematoxylin solution to act upon 

 the section, the layers around the cell-cavities stain a deep violet, 

 while the whole jelly takes a bright violet tone and becomes every- 

 where readily visible. 



Growth in Thickness of Fucus. Even \vith the naked eye 

 w r e observed that the thallus in its older parts is reduced to the 

 midrib, expanded into a strong stern. The growth in thickness 

 which brings about the development of this stem out of a midrib 

 takes place in the innermost layers of the cortex. The cells of 

 these layers develop from their lower end sac-like prolongations, 

 which, dividing by cross-walls, and branching from time to time, 

 grow downwards in the jelly between the threads of the pith. 

 Only here and there one of these thickening threads grows into 

 the packing tissue of the wings. Longitudinal sections through the 

 neighbourhood where noticeable increase of thickness of the midrib 

 begins, shows us these phenomena without any difficulty. If we 

 examine the cross-section of the deeper parts of the stem, we find 

 it constructed internally of sparsely-scattered cells, with wide 

 cavity and brownish contents, and in between these of numerous 

 closely-crowded cells, with narrow cavity and greenish contents. 

 The former are the original threads of the medula, which serve for 

 food conduction ; the latter are those which have been intruded in 

 growth in thickness, and have to fulfil mechanical functions. The 

 threads originally present have been pushed apart by those formed 

 later. The base of the stem and the attaching disk are, in fully 

 developed specimens, exclusively composed of these mechanically 

 acting threads. In the outer part of the stem the tissue has also 

 undergone a change. The cells of the outer layer have become 

 brown, are dead, and are gradually cast off. The second layer 

 of the cortex has begun to divide by periclinal walls. We find 

 therefore in the exterior parts of the stem radially-arranged rows 



