Lecture XIV. 105 



be called the cuticle. Within the superficial layer the cells of 

 the cortex are larger and they increase in size from without 

 inwards. They are polyhedral and they measure approximately 

 the same in every direction. (Cells with this shape are called iso- 

 diametrical.) Like the superficial cells they have thin walls and 

 they appear completely filled with protoplasm, in which is em- 

 bedded a single nucleus and countless very minute chromatophores. 

 These latter contain the olive-brown colouring matter which gives 

 the colour to the frond. Like the chloroplasts of the green cells 

 we have examined, the chromatophores are made of protoplasm 

 and they contain the four pigments found in the chloroplasts, viz. 

 chlorophyll a and b, xanthophyll, and carotin. In addition to 

 these four there is in them another brownish pigment, fuco- 

 xanthin. While in the green plants the quantity of the two 

 chlorophylls present in the chloroplasts is about five or six times 

 that of the yellow pigments, in the chromatophores of Fucus the 

 ratio of the yellow plus fucoxanthin to the chlorophylls is 12/17. 

 This difference accounts for the difference in colour. The 

 presence of the chlorophyll may be very easily demonstrated by 

 immersing the frond in hot water. This causes the yellow pig- 

 ments to bleach and the undiluted green of the chlorophyll 

 present becomes apparent. The same result may be obtained 

 by a short immersion in spirit. 



As might be expected, the chlorophyll in these cells is present 

 to enable the plant to form carbohydrates from the carbon dioxide 

 in the surrounding water. When Fucus has been exposed to 

 bright light a granular substance is found in these cells. It is 

 probably a carbohydrate. Thus we realise that Fucus derives its 

 carbon by photosynthesis in a manner similar to other plants 

 possessing chlorophyll. 



The inner layers of the cortex are composed of cells with more 

 irregular shapes than those of the outer layers just described. 

 They are elongated in one direction and have thicker cell-walls. 

 Their protoplasm does not completely fill the cavity of the cell 

 and a vacuole of considerable dimensions is seen. The chromato- 

 phores are not so densely crowded in the protoplasm. The 

 change from the outer to the inner cortex is not abrupt but the 

 characters of the cells of the two layers grade into one another. 

 The central region of the frond is called the medulla. It is com- 

 posed of strands of elongated cells which resemble hyphae, and 

 cohere loosely together at irregularly spaced points of con- 

 tact, leaving irregular spaces between them. Branches from the 

 cells of the inner cortex penetrate between the hyphse of the 



