MUL TIN UC LEA T.E 



283 



the germinating tube which springs directly from a zoospore. Most of 

 the species are monoecious, and the oogones and antherids are usually 

 found very near together. The antherids are the terminal portions of 

 slender branches, in some species straight, in others curved and more 

 or less resembling horns or hooks. They contain but little chlorophyll, 

 and the protoplasm breaks up into a large number of biciliated anthero- 

 zoids, which escape through the ruptured apex. The two cilia are of 

 unequal length, and point, one backwards, the other forwards. The 

 oogones arise as thick swellings, often somewhat resembling a bird's 

 head in shape, and are densely filled with chlorophyll ; they are finally 

 cut off from the rest of the tube by a septum. The coarsely-granular 

 green protoplasm collects in the centre of the oogone, leaving a colour- 

 less portion at the apex which corresponds to the beak of the bird. 

 Here the oogone opens, and a colourless drop of mucilage is sometimes 

 expelled. When the greater part 

 of the contents has contracted 

 into an oosphere, a number of 

 .antherozoids force their way in 

 through the open mouth of the 

 oogone, and impregnate the 

 oosphere by coalescing with it. 

 The oosperm, resulting when the 

 impregnated oosphere has be- 

 come invested by a cell-wall of 

 cellulose, assumes a red or brown 

 colour, and passes through a 

 period of rest as a hypnosperm. 

 On germination several non- 

 sexual generations are pro- 

 duced, ending in the formation 

 of sexual organs. The formation of antherids and oogones begins 

 in the evening and is completed the next morning ; fertilisation being 

 usually accomplished between 10 A.M. and 4 P.M. It is most usual for 

 zoospores to be formed on parts of the thallus which are completely 

 submerged in water ; while the sexual organs are more commonly found 

 when the plant grows on moist ground or on the margin of a ditch. 



Several species of Vaucheria are frequent in fresh, while some occur 

 .also in brackish or salt water ; others abound on moist or shady ground, 

 being especially common in flower-pots or on neglected gravel-paths, 

 where they form light green tufts or thick mats. The normally uni- 

 cellular thallus is liable to segmentation as the result of injury ; and, 

 even when uninjured, has a tendency to become septated by thick 



FIG. 249. Vaucheria dichotoma Lyng. A, 

 oogone; , germinating spore (x 200). (After 

 Woronin.) 



