STRUCTURE OF FUCUS 229 



{Laminaria) of our own coasts, some of the medullary 

 cells are very highly specialised for conduction of 

 organic substances, and are strikingly like, even in 

 small details, the sieve tubes of the higher plants, 

 described in Chapter XVI. 



PRACTICAL WORK. 



Fucus. 

 Vegetative Structure. 



(i) Make a sketch of samples of the thallus of Fucus, showing 

 the holdfast, the stipe, the branching, the positions of the apical 

 grooves, and the conceptacles. Note that the stipe is extremely 

 tough and cannot be broken by pulling with the hands, whereas 

 the frond can. If the frond is broken the hair-like fibres may 

 be seen projecting from the broken surfaces. 



(2) Examine a cross-section of the frond under the low power 

 and draw a diagram ' of the general plan of distribution of the 

 tissues, marking : — 



(o) the palisade [photosynthetic) layer on the surface, 



(6) the cortex of large isodiametric cells, with fewer phseo- 



plasts per unit volume, 

 (c) the medulla of elongated cells running in strand in various 

 directions and separated by a matrix of cell wall sub- 

 stance. 



Note that the extra thickness of the midrib is caused by the 

 greater thickness of the medulla in that region. 



(3) Examine a cross-section of the frond under the high power, 

 and make careful drawings ' of small samples of the various tissues, 

 including the thick-waUed fibres ("hyphae "), which are mainly 

 localised just below the cortex of the midrib, on the edge of 

 the medulla. Note that the conducting cells run horizontally 

 or obliquely in the wings, longitudinally (so that they are cut 

 transversely) in the midrib. 



(4) Examine two longitudinal sections of the frond (a) cut 

 through the midrib at right angles to the surface of the frond, 

 (b) cut parallel to the surface of the frond through the centre. 

 Identify the various tissues already seen in transverse section, 

 and from a comparison of the appearance of the cells in the 



' See p. 19. " See p. 20. 



