DIFFERENTIATION OF SURFACE AND INTERNAL CELLS 227 



the surface in the centre of the upper end, and by the 

 more rapid growth and division of the neighbouring 

 surface cells the apical cell shortly becomes sunk in 

 a hollow. Up to this point the young plant is nearly 

 cylindical in form, but now the upper part grows more 

 quickly in one longitudinal plane than in the plane 

 perpendicular to it, and thus the wings of the frond 

 are started. 



A marked difference is soon apparent between the 

 growth of the surface cells and those occupjdng the 

 centre of the thallus. The former remain small, 

 divide actively and form the pahsade (surface) layer. 

 The cells lying immediately below do not divide so 

 rapidly and form the cortex. The central cells are still 

 more sluggish and become passively stretched in the 

 longitudinal direction as the surface increases in 

 extent by active growth and division of the outer 

 cells (Fig. 33, A), At the same time the middle layers 

 of their walls become mucilaginous and separate the 

 bodies, central cells from one another laterally, thus 

 giving rise to the characteristic structure of the 

 medulla.' 



This same contrast between outer and inner cells 

 can also be seen in other Brown Seaweeds (not at all 

 closely allied to Fucus) whose thallus consists of stout 



Fig. 35. — ^A, developing oogonia from surface of conceptacle with 

 accompanying hairs. B, body cell of oogonium containing eight 

 young eggs (female gametes). C, freeing of the eight eggs by 

 bursting of the bladder surrounding them. D, single egg (female 

 gamete) surrounded by swarming male gametes. E, fertihsation. 

 The nucleus of a sperm (male gamete) (sp.n.) has penetrated the 

 egg and is approaching its nucleus. F, sperm nucleus in contact 

 with wall of egg nucleus. G, sperm nucleus ingide egg nucleus. 



" The hypha-like chains of thick-walled cells (fibres) which form 

 the whole of the holdfast, the greatest part of the medulla of the 

 stipe, and also appear in the midribs of the fronds, begin to grow out 

 from the cortical and the medullary cells as soon as these separate, 

 and the " hyphae ". grow independently in the mucilaginous matrix. 



