1 1 6 Practical Plant Biology. 



persists. Thus the protoplasm of the cortical cells of the 

 successive branch segments become connected and the number 

 of the nuclei in these cells is augmented. 



Branches are formed close to the apex. They arise by the 

 formation of a lateral bulge on one of the disc-shaped segments 

 beneath the apical cell. This becomes cut off from the disc- 

 shaped cell and acts as a hemispherical cell. If longitudinal 

 divisions do not quickly supervene it becomes a hair : if they occur 

 it is called a branch. No chromatophores are found in the cells 

 of the hairs. 



Carbohydrates are formed in the cortical cells of the branches 

 by the chromatophores which contain chlorophyll and accessory 

 pigments. They are often stored there, and in the axial cells, in 

 the form of the starch-like bodies already mentioned. The 

 necessary salts, especially nitrates, and water appear to be absorbed 

 all over the surface ; possibly the hairs are specially effective in 

 this absorption. The primary protoplasmic connections of the 

 axial cells convey proteins elaborated from the carbohydrates and 

 mineral supplies to the apical cell and the growing region immedi- 

 ately below it. The slower growth of the basal parts and the 

 repair of their waste is provided for by the radial connections and 

 the secondary connections of the cortical cells. The small size of 

 the plant renders the development of a special conducting tissue 

 such as is found in Fucus unnecessary. For respiration Polysi- 

 phonia utilises the oxygen dissolved in the sea-water. It is there- 

 fore aerobic. 



Polysiphonia has two very distinct methods of reproduction, an 

 asexual and a sexual process. They are of great interest as 

 elucidating the life-history of higher plants. 



Asexual reproduction is effected by means of tetraspores. These, 

 as their name indicates, are formed in fours in a mother-cell the 

 tetrasporangium. The tetrasporangium arises as follows : one of the 

 cortical cells of a segment near the apex divides longitudinally into 

 an inner and an outer segment. The inner segment divides trans- 

 versely into a large upper cell and a small basal one. The upper 

 one grows and swells into a large spherical cell, which is the 

 tetrasporangium. Its nucleus divides into four, and the cytoplasm 

 cleaves into four masses each containing one of these nuclei. 

 These cells are the tetraspores. They are arranged tetrahedrally 

 in the sporangium. At first, owing to their mutual pressure, they 

 each have three flat pyramidal faces. Later, as the wall of the 

 sporangium stretches they become spherical. They then escape 

 by the breaking up of the branch in which they are formed. 



