222 DIFFERENTIATION OF TISSUES. FUCUS 



sarily the strands of medullary cells, because these 

 form the shortest paths, and also, owing to the length 

 of these cells, there are fewer walls to pass in a given 

 distance. The medullary cells are connected at one 

 end with the cortex, and through it with the palisade 

 layer of the mature frond ; at the other with the mass 

 of embryonic cells lying behind the apical cell. 

 Hence these medullary cells may be called conducting 

 cells. The diffusion of substances from a region of 

 higher to a region of lower concentration, in plant 

 and animal tissues for instance from a region of 

 constant production to a region of consumption (i.e. con- 

 version into other substances), is a necessary conse- 

 quence of the physical laws of diffusion, though the 

 rate of diffusion and the actual paths it takes will 

 depend on a number of variable factors, such as the 

 nature of the substances and the obstacles to diffusion. 

 The cortical cells will act to a certain extent as a 

 storage tissue, since if the stream of soluble organic 

 food from the photosynthetic layer through the 

 cortex and medulla to the growing points is checked, 

 owing to the supply being temporarily greater than 

 the demand, some of the surplus food will be arrested 

 and will tend to accumulate in the cortical cells as 

 well as in the medullary cells. 



Sexual Reproduction. Unlike the plants hitherto 

 considered Fucus reproduces itself exclusively by 

 means of gametes, and these show high sexual dif- 

 ferentiation, comparable with that of the gametes of 

 Volvox (cf. Figs. 28, G, and 35, D). The two kinds of 

 gametes are produced in the cells of the sexual organs 

 arising on the inner surfaces of the conceptacles. In 

 some species the male and female organs are formed 

 in the same conceptacle in others in different concep- 



