Lecture XIV. 107 



the ends of the groove form the flattened parts of the frond on 

 each side of the mid-rib. These latter persist only for a time and 

 as the apex grows and forms new tissues the older tissues at each 

 side of the mid-rib gradually wear away and leave the mid-rib 

 alone persisting. The mid-rib thus stripped of the lateral 

 parts of the frond persists and forms the cord-like stem of the 

 plant. 



The localisation of the growth of the plant at definite spots, e.g. 

 the tips of the fronds, necessitates a constant supply of nutritive 

 materials to these places for the manufacture of these new cells. 

 The supply is conveyed by the elongated cells of the mid-rib from 

 the cells of the cortex all over the plant. 



We have seen that the cortical cells manufacture carbon com- 

 pounds by photosynthesis ; they also especially the tufted hairs 

 may be supposed to absorb nitrates and other salts from the sea- 

 water. These materials, either in the crude form or more prob- 

 ably after being synthesised into proteins, are transmitted from the 

 outer cortex through the elongated cells of the inner cortex into 

 the medullary hyphse and so into the mid-ribs where they are 

 conveyed in the parallel hyphae to the growing points. 



In Fucus then we have a marked division of labour among the 

 tissues, some carrying on absorption, some photosynthesis, some 

 acting as channels of communication. In addition to these we 

 have special groups of cells set aside for growth, a special part 

 again acting as an organ of attachment, and, as we shall see, all 

 these somatic cells are different from the reproductive cells. 

 Such physiological specialisation, as is invariably the rule, is 

 associated with structural differentiation. Evidently such struc- 

 tural differentiation is impossible in unicellular organisms in 

 which the single cell has to discharge a number of functions 

 simultaneously. So that it is only in multicellular organisms we 

 can expect to find marked division of labour and with it differences 

 in form and structure (morphological and structural differentiation) 

 among the individual cells. 



The reproductive cells of Fucus are very markedly differenti- 

 ated from the somatic. They are formed in the cushion-like 

 terminations (receptacles) of some of the branches. The re- 

 ceptacles are studded over with a number of wart-like prominences 

 from which, as we have seen, delicate hairs emerge. The opening 

 through which the hairs emerge leads into a small spherical cavity, 

 called a conceptacle, formed in the cortex of the frond. The 

 cells composing the wall of this conceptacle support the hairs just 

 mentioned ; among the bases of the hairs the special capsules for 



