430 PRACTICAL BOTANY 



peculiar form, which are the organs of sexual repro- 

 duction (gametangia), viz. 



a. Curved cylindrical bodies, which are the an- 

 theridia. 



~b. Obliquely oval, sessile bodies which are the 

 oogonia. 



In this species the sexual organs are associated 

 together in groups of two or three, each being inserted 

 separately upon the thallus : a single antheridium is 

 usually associated with one or two oogonia. 



It will be necessary to distinguish the species above named both 

 from other organisms not nearly related to it, and also from other 

 species of the same genus. Thus, the Moss protonema is usually 

 found growing on the surface of pots, but it is readily distin- 

 guishable by its septa and mode of branching : other septate fila- 

 mentous Algae may also be associated with it, but are easily 

 distinguished by their size, colour, septa, &c. Of other species of 

 the genus Vaucheria, that most likely to be present is V. terrestris, 

 which may be distinguished from V. sessilis by the antheridia and 

 oogonia being inserted together on a common pedicel : other 

 species differ in the insertion, number, and form of the antheridia 

 and oogonia. 



II. Put on a high power, and examine the structure 

 of the thallus in detail. Note 



1. The smooth continuous external cell-wall : this 

 may be made more apparent by plasmolysing some 

 filaments with a 2 per cent, solution of common salt. 



2. The protoplasmic membrane, which lines the 

 wall, and incloses a large central vacuole which runs 

 the whole length of the filament : this membrane 

 may also be more readily distinguished in plasmolysed 

 specimens. In the membrane are imbedded 



3. Numerous oval, or spindle-shaped cholorphyll 



