LECTURE XV. 



POLYSIPHONIA is one of the Red sea-weeds. In general its species 

 are small and inconspicuous, being at most a few centimetres in 

 height, but it makes up for its diminutive size by its beauty of 

 colour and of structure. 



Our study will be of Polysiphonia fastigiata which is always 

 found growing on one of the Brown sea-weeds (Ascophyllum 

 nodosuni), a large Fucus-like plant with a narrow frond and large 

 and conspicuous bladders. Polysiphonia appears like tufts of 

 coarse dark red-brown hair over this plant. Although it always 

 grows on this plant, apparently it uses the brown weed only as a 

 support and is not parasitic upon it. Technically it is an epiphyte. 



When submerged in water Polysiphonia shakes itself free and 

 appears as a delicate filiform plant, of an exquisite deep rose 

 colour, the branches of which taper out into great fineness. 

 Examined microscopically each branch is seen to be cylindrical 

 and composed of segments. Each segment is made up of an 

 axial cylindrical cell surrounded by about eighteen prismatic cells 

 which form a cortex round it. All these cells are of the same 

 height. The axial cell, which is somewhat bulged at its equator, 

 and so somewhat barrel-shaped, has a diameter about equal to its 

 height. The radial sides of the prismatic cells are as high as the 

 axial cell but measure radially only about one-fourth or one-third 

 their height. Owing to their wedge-like form their outer tangential 

 wall is about twice as wide as their inner wall and its width equals 

 about \ of its height. The upper and lower ends of the cortical 

 cells are roof-shaped the ridge running radially in the stem, 

 and fitted between the adjacent ends of the cortical cells of 

 the segment above and below. Owing to this bevelling of the 

 ends of these cells their outer tangential walls are elongated 

 hexagons when viewed from outside. Often a slight spiral twist 

 of the branch imposes on these cells a distortion, which relieves 

 the regularity of construction from geometrical severity. The 

 distinct outlines and graceful proportions of the cells and their 



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