58 



PLANT MORPHOLOGY 



some live in the ocean. The plant body consists of a sparsely branched 

 coenocytic filament without any cross walls in the vegetative portion. It 

 is attached by means of colorless rhizoid-like holdfasts. Numerous small 

 nuclei and chloroplasts are scattered throughout the cytoplasm, which 

 surrounds a large central vacuole. There are no pyrenoids or starch 

 grains, but oil droplets are usually present in abundance. In this respect 

 Vaucheria differs from the other Siphonales. 



Vaucheria displays three methods of vegetative reproduction, as 

 follows: (1) A branch may be constricted at the base, thus producing a 



>^ 





T^i 



s^V*^ 



i^p. 





A C 



Fig. 46. Fawc/ieria, a coenocytic green alga, X250. A, an escaping zoospore covered with 

 many cilia; B, a zoospore giving rise to a new vegetative filament; C, two oogonia of 

 Vaucheria sessilis, each with a zygote ; also an antheridium that has discharged its sperms ; 

 D, two sperms, more highly magnified. 



new plant body directly. (2) The tip of a branch may swell slightly and 

 become cut off by a cross wall to form a club-shaped sporangium (Fig. 

 46A). The multinucleate protoplast in the branch rounds up and 

 becomes a large zoospore entirely covered by cilia. The cilia are in pairs 

 and beneath each pair is a nucleus. For this reason the zoospore is 

 regarded as compound. It escapes into the water through a terminal 

 pore and, upon germination, gives rise to a new filament (Fig. 465). (3) 

 The contents of an entire filament may break up into aplanospores, each 

 developing a thick wall. 



Vaucheria is heterogamous. The antheridia and oogonia are not 

 transformed vegetative cells but are developed on special branches of the 

 filament (Fig. 46C). In most species a short branch, sooner or later cut 

 off by a wall, becomes a globular oogonium. Its protoplast is organized 

 as an egg, which becomes uninucleate. It is uncertain whether this is 



