2l8 DIFFERENTIATION OF TISSUES. FUCUS 



water and swells, forcing the cells apart and increasing 

 the thickness of the fhallus. The layers of wall on each 

 side of this swollen middle layer, i.e. in direct contact 

 with the cell bodies, also increase in thickness, more 

 or less, but remain of firmer consistency, so that in 

 the adult condition the cells appear isolated, each 

 covered by a wall of its own (which may be thin or 

 thick) and separated from its neighbours by a mucila- 

 ginous matrix. The medullary cells form chains or 

 strands (likei the threads of a filamentous alga) of 

 cylindrical cells placed end to end. It is these chains 

 which are separated from one another by the mucila- 

 ginous matrix derived from the middle layer of the 

 original cell wall (Fig. 33, A). The cross walls separating 

 the successive cells of a medullary strand remain thin. 

 The structure of the body of a medullary cell is nbt 

 different in essentials from that of a cortical cell, that 

 is to say there is a central vacuole and peripheral 

 cytoplasm with nucleus and phseoplasts, but these 

 last are often very sparsely scattered. 



The direction of the medullary strands differs as 

 between the wings and the midrib : in the former they 

 run horizontally or obliquely, in the latter longitudin- 

 ally, so that they appear in cross-section as circles. 

 Groups of cells at the outer edge of the medulla of 

 the midrib, just below the cortex, have especially 

 thick tough walls, and these may be called fibres, by 

 analogy with the somewhat similar longitudinally 

 running thick walled cells of the higher plants. Like 

 theiji they increase the toughness of the thallus. The 

 main function of the medullary cells is probably that 

 of conduction of . organic food substances formed by 

 the photosynthetic layer to the regions of growth at 

 the apex__^of the frond. 



