STRUCTURE OF STIPE AND APEX 219 



In longitudinal sections of the frond the palisade 

 and cortical cells appear very much the same as they 

 do in transverse sections, but the medullary cells of 

 course look different, those of the midrib being now 

 cut in longitudinal section, i.e. showing the length of 

 the chains, instead of as single cells cut transversely. 



(2) Stipe. — The stipe consists of cells similar to those 

 of the frond, but with some notable differences. There 

 is no single layer of surface cells overlying a distinct 

 cortex, but several layers of cells in radial rows, the 

 cells of each row separated by thin walls, indicating 

 division parallel to the surface of the frond {tangential 

 division). The surface itself is rough owing to the 

 knocking about the stipe suffers from the waves 

 against the rocks, and often shows radial splits. The 

 medulla, which forms much the greatest bulk of the 

 stipe, consists largely of fibres, with a few wide cells 

 having dense vacuolated contents, including a number 

 of phaeoplasts and fairly thick walls. These may 

 perhaps serve to conduct organic food from the frond 

 down to the holdfast, which increases in size, sending 

 out fresh short branches as the plant grows, As the 

 base of the stipe is approached the thick-walled fibres 

 increase and the holdfast itself consists exclusively of 

 them. 



Apical Growth and Nutrition. — The basis of growth 

 in Fucus, as in the higher plants, is active cell divi- 

 sion at the tips of the branches. At the bottom of 

 the apical groove of each branch there is situated a 

 comparatively large six-sided, box-like cell (Fig. 33, A, a), 

 with one side forming part of the surface of the base 

 of the groove. This apical cell gives rise by its divi- 

 sions to the cells of all the tissues of the branch. The 

 apical cell is constantly dividing unequally, cutting off 



