REPRODUCTION. 41 



With high power, note more especially the crenulated and 

 wrinkled margin of the bands and the refractive nodules at 

 intervals along them (pyrenoids). Look for a nucleus. This 

 is sometimes hidden by the chlorophyll bands and then not 

 easily seen, but there will always be found some cells in which 

 the nucleus stands out clearly. 



Apply iodine solution. The bands stain deeply, especially 

 the dense pyrenoids, which appear almost black. The nucleus 

 also appears much more conspicuous when stained. 



It may not be possible to obtain sprirogyra in a growing state 

 at the time it is wanted for study. In such a case it should be 

 collected at some other time, when a supply is found, and pre- 

 served for future use in dilute formaldehdye solution (one vol- 

 ume of 40 per cent, "formalin" to 10 or 15 volumes of water). 

 In this solution the color is bleached only slowly, and if the 

 specimen has not been kept too long it will still show a green 

 color. 



Another large-sized alga often found in slow streams is the 

 Hydrodictyon or Water-net. The cells are united into network, 

 which has the shape of an elongated bag, often 8 or 10 inches 

 long. The cells are quite large, cylindrical and filled with 

 dense granular chlorophyll matter. The ends of three or four 

 cells meet at a common point. Several nets may be entangled 

 in one another, representing different generations, the cells of 

 the younger nets being much smaller than those of the old and 

 matured nets. 



The structure is best studied under low objective in a watch 

 glass containing water, as thus crushing of the net is avoided, 

 which would happen under a cover glass. 



CHAPTER VI. 



REPRODUCTION. 



This is the power that plants possess of giving rise to new 

 individuals, and the process takes place by one of three ways, 

 namely, Division, Rejuvenescence and Union. The first two 

 modes are asexual, the last sexual. 



There are three varieties of reproduction by division : 



fFission. 

 DIVISION^ Gemmation. 



[internal cell formation. 



FISSION. The most common mode of division. This is the 

 separation of a cell into equal portions. 



a. A constriction takes place in the middle of the cell and 

 along the plane of this constriction ; the cell-walls may grow 



