VAUCHERIA 67 



possess numerous cilia, enabling them to swim; but 

 they are active for a very short time, and only in a 

 sluggish fashion. They soon come to rest and germin- 

 ate, a filament being produced which sends a root process 

 into the soil and forms branches rapidly. 



Asexual reproduction by zoospores is the normal 

 order of things with Vaucheria, so long as water is plenti- 

 ful and the season is favourable. But if water tends 

 towards scarcity, the plant anticipates drought, and 

 prepares to tide over adverse conditions by forming 

 fertilized egg-cells. The story is illustrated in Plate II. 

 Two organs are formed, the antheridium (a;) and the 

 oogonium (o). In the antheridium, which becomes 

 separated from the process in which it is developed by 

 a septum, a number of minute flagellated spermato- 

 zoids (z) appear; while a passive egg-cell, large and well 

 nourished, is formed in the oogonium. The spermato- 

 zoids duly escape from the antheridium, and swarm 

 round the egg-cell, finding their way into the oogonium 

 through an aperture. One individual, more fortunate 

 than the rest, fuses with the egg-cell and fertilizes it. 

 The fertilized egg-cell secretes a stout cell -wall around 

 itself, and is now prepared to endure even a prolonged 

 time of drought. It germinates when conditions are 

 favourable, and forthwith produces an ordinary thallus, 

 which is not long in forming zoospores, and continues to 

 form them for several generations. 



Thus Vaucheria, which may be called a non-cellular 

 Alga, is remarkable in two particulars : it is very simple 

 in its vegetative arrangements, yet, in spite of that 

 simplicity, it has developed sex-difierentiation of a high 

 order. As we have already indicated, the Siphonese do 



