21.6 DIFFERENTIATION OF TISSUES. FUCUS 



raised papillae, each of which is the projecting top of 

 a spherical structure, hollow within, called a coth 

 ceptacle. A minute hole leads through the tip of the 

 papilla into the hollow of the conceptacle, which con- 

 tains the sexual organs bearing the highly differentiated 

 gametes. There are also (often) smaller, sterile con- 

 ceptacles from which hairs protrude through the 

 opening to the outside. 



Thus in Fucus we have an external differentiation 

 of parts or organs, though a very simple one : the 

 holdfast which fixes the plant below, the stipe which 

 is specially tough, and the frond which plays the chief 

 part in photosynthesis and in growth ; while the 

 sexual reproductive organs are represented by groups of 

 cells arising on what is really the surface of the frond, 

 though the surface locally dips down, so to speak, to 

 form the internal surface of the hollow conceptacle. 



Microscopic Stracture of the Thallus.^The minute 

 structure of the different organs of bulky plants is 

 best studied by examining under the microscope 

 sections of the organ thin enough to be translucent 

 when mounted in a liquid medium. To obtain com- 

 plete information as to the structure of such an organ 

 these sections have to be cut in different directions. The 

 most instructive section is that taken at right angles 

 to the axis of symmetry of the organ (transverse or 

 cross section), for this displays the distribution of 

 tissuef about that axis. But the knowledge we gain 

 from a transverse section must be supplemented by 

 the examination of sections taken through and parallel 

 with the axis (longitudinal sections), in order to study 

 the structure of the tissues in longitudinal extension. 



(i) Frond. — A cross-section of the middle of the 

 frond shows three clearly marked regions : (a) the 



