SPIROGYRA 207 



tinctioii between somatic and germ cells is not an 

 absolute one. Though the germ cells (spores and 

 gametes) are the specialised reproductive cells, whose 

 sole function is to reproduce the species, this power 

 may be retained by the body cells to a varying extent 

 in different organisms. The body cells of plants retain 

 it far more generally than those of animals, and this 

 is undoubtedly connected with the fact that plant 

 cells are, in general, far less highly modified for the 

 performance of special functions than is the case with 

 the cells of the higher animals. 



Spirogyra.-^This is a filamentous green alga belong- 

 ing to a group which is distinguished by producing 

 gametes that are not flagellated cells, conjugation 

 taking place entirely within the mother cell walls. The 

 body consists of an unbranched thread composed of 

 cylindrical cells placed end to end. Each cell has a 

 large central vacuole and a thin layer of cytoplasm 

 lining the wall : in this is embedded a single chloro- 

 plast in the form of a green band which winds 

 spirally round the cell from end to end (Fig. 31, A). 

 In the chloroplast is a row of pyrenoids. The nucleus 

 is suspended in the centre of the cell. In other species 

 several chloroplasts are present, running parallel with 

 one another. The crossing lattice structure seen when 

 the cell is looked through from the side is due 

 to the fact that the parts of the chloroplasts which 

 run round the further side of the cell are seen at the 

 same time as the parts on the side towards the observer, 

 so that' the former appear to cross the latter. In a 

 wide cell containing several chloroplasts, when the 

 near side of the cell can be focussed alone, the far 

 side being out of view, it is clear that there is no actual 

 crossing (Fig. 31, B). In such a large cell the nucleus 



