22 PLANT-LIFE ON LAND [OH. 



place of four. They differ also in their behaviour, 

 for it has been observed that the gametes which 

 escape from different filaments when they meet 

 coalesce in pairs (Fig. 3 A />). The result of the 

 fusion is a cell called a zygote (E). It soon loses its 

 cilia, takes a rounded outline, and forms a cell- wall (F). 

 After a period of rest the zygote germinates, bursting 

 its cell-wall, and the contents divide into four parts 

 (F, 6r), each of which may grow into a new filament 

 like the parent. The propagative methods in Ulva 

 and other related Algae are similar to those in Ulo- 

 thrix, but they have not been followed out with the 

 same exactitude. It has been shown by Professor 

 Klebs that the various forms of the motile propagative 

 cells do not follow in any necessary sequence. It 

 would seem probable that their differences are cor- 

 related with external conditions, and that it is tiiese 

 which decide in what sequence they shall appear, 

 rather than any internal necessity. 



Such a story of propagative method as this in 

 UlothriXj combined with the similar facts which 

 result from the study of other allied forms, leads 

 to various general reflections. In the first place it is 

 seen that, in the cycle of life of Ulothrix or of Ulva, 

 two phases of existence appear. The one is stationary, 

 a condition imposed upon it by the fact that the living 

 protoplasm of each cell is encysted, that is confined 

 within a cell-wall, which makes free movement 



