220 DIFFERENTIATION OF TISSUES. FUCUS 



a flattish oblong cell from each of the four sides and 

 from the base (towards the centre of the frond) in 

 turn. After each division it grows to its original 

 size. The cells cut off divide further and form an 

 almost homogeneous small-celled thin-walled tissue, 

 densely filled with phseoplasts, in the whole region of 

 the branch apex. As one passes away from the apex 

 the different tissue regions quickly differentiate 

 (Fig- 33> A). The surface cells do not change their 

 character much and form the photosynthetic layer. 

 The cells lying next below increase in size by swelling 

 of the vacuole, but do not increase their cytoplasm. 

 These cells become the cortex. The cells towards the 

 centre of the frond grow in length very considerably 

 and separate laterally by the swelling of the middle 

 lamellae of their lateral walls, as already described, 

 thus forming the medulla. The strands of medullary 

 cells are also passively stretched owing to the elonga- 

 tion of the frond by the continued division of the 

 surface cells at right angles to the surface plane. 



It is clear that at the apices of the branches there 

 is constantly going on a great increase in the bulk of 

 protoplasm and of cell wall substance, and this requires 

 a continuous supply of soluble carbohydrates and 

 proteins or other nitrogenous organic substances. 

 Some of this organic food will no doubt be supplied 

 by the synthesis of these substances from carbon 

 dioxide, water and salts absorbed by the surface layers 

 of cells close to the apex, which contain numerous 

 phaeoplasts. But in an actively growing plant the 

 consumption will be much greater than this supply, 

 and the balance required must come mainly from the 

 palisade layer of the mature parts of the frond. The 

 easiest channels for this flow of substances are neces- 



