Lecture IX. 69 



which shows this motion. Each chloroplast is ribbon-shaped, 

 slightly grooved on its outer and bowed on its inner surface. 

 There is a thickened rib running down its middle-line. Its 

 margins are irregularly indented. The mid-rib appears thickened 

 at regular intervals into little knobs consisting of a protein mass 

 surrounded with starch grains. These are pyrenoids. By suit- 

 able microscopic manipulation it will appear that the chloroplasts 

 are wholly embedded in the cytoplasm and that this latter sur- 

 rounds a large central vacuole, containing a clear colourless fluid. 

 In the centre of the vacuole one can often plainly see a nucleus 

 suspended. It is a lens-shaped capsule formed by a nuclear 

 membrane enclosing a drop of clear fluid in which is suspended a 

 highly refringent speck, the nucleolus. Generally in healthy cells 

 the axis of the cylindrical cell coincides with the axis of the 

 lenticular nucleus, but a slight mechanical injury or chemical 

 stimulation causes it to rotate so that its axis is at right angles to 

 that of the cell and it presents a circular outline to the observer. 

 It will be noticed that a thin film of cytoplasm adheres to the 

 outer surface of the nuclear membrane and from the capsule so 

 formed enclosing the nucleus very fine filaments of cytoplasm ex- 

 tend and connect the capsule with the cytoplasmic lining of the 

 cell-wall. It will be noticed that most of these filaments may be 

 traced to the cytoplasm in the immediate neighbourhood of a 

 pyrenoid. 



So far as is known the metabolism of a cell of Spirogyra is very 

 similar to that of Chlamydomonas. When exposed to light and 

 when the water which surrounds it is charged with carbon dioxide, 

 as is normally the case, the evolution of oxygen bubbles shows 

 that photosynthesis is being carried on. During this time suitable 

 observations show that the starch in the pyrenoids increases. 

 Spirogyra lives in water that is free from organic nitrogenous 

 compounds and it evidently obtains its nitrogen from the traces 

 of dissolved nitrates in the water surrounding it. Hence its 

 nutrition with regard to carbon and nitrogen is typically holophytic. 

 Its respiration is aerobic and if the water in which it lives is 

 deprived of dissolved oxygen, Spirogyra dies. 



Possessing a complete cell-wall Spirogyra has no contractile 

 vacuole and hence no special apparatus which eliminates dissolved 

 substances including waste-products. Such active elimination 

 seems rendered unnecessary by the very economical metabolism 

 which Spirogyra possesses in common with the majority of the 

 vegetable kingdom. Thus the waste-products of its catabolism, 

 both nitrogenous and carbonaceous, act as the raw materials of 



