and Laboratory Methods. 2055 



LABORATORY OUTLINES. 



For the Elementary Study of Plant Structures and Functions from 

 the Standpoint of Evolution. 



A SERIES OF FORMS TO ILLUSTRATE THE EVOLUTION OF SEX. 



XVIII. Vaucheria sessiiis Vauch. Class, Siphoneae. Order Vaucher- 

 iales. Family, Vaucheriaceae. 



This alga grows as a lax, green, felt-like layer on the surface of moist soil, 

 and is especially common on the surface of pots in greenhouses. Other species 

 may be found in ponds. 



1. Describe the naked eye characters, noting the coarse cylindrical filaments. 



2. Under low power draw an entire filament showing the branches, the tips, 

 and the decaying part of the back end. Note the absence of transverse walls. 

 Such a plant body is called a coenocyte. How does the filament grow ? 



3. Under high power draw a short piece of a young filament, showing 

 details — shape, vacuole, arrangement of protoplasm, chloroplasts, and oil drops. 

 Position of chloroplasts ? How can you tell that the filament is cylindrical with- 

 out seeing a cross section ? 



4. Draw several chloroplasts. Shape ? Draw some in stages of division. 

 Describe. Look for movement of the protoplasm. Numerous nuclei are present 

 in the cytoplasm, but these are not visible without special staining. 



5. Study the sexual organs, the antheridium (spermary) and oogonium 

 (ovary). They are usually side by side. Draw carefully and describe. Notice 

 the septa which separate the sexual organs from the main filament. Vaucheria 

 is hermaphrodite, having male and female organs on the same individual. 



6. Draw the oosphere (unfertilized Qgg) ; also some spermatozoids in the 

 antheridium. Look for free-swimming or escaping spermatozoids ; also for 

 spermatozoids entering the oogonium. 



7. From the union of the two gametes an oospore is formed. Draw a ripe 

 oospore showing the thick wall and more or less hyaline contents. Describe. 



8. Contrast the two sex cells (gametes) as to size, motion, and nutrition. 

 How is an oospore different from a zygospore ? Would there be any advantage 

 in this ? 



9. Special vegetative propagation by means of volvox-like colonies (com- 

 pound zoospores), produced in the ends of the filaments, may be obtained as 

 follows : Place a mass of Vaucheria in a porcelain dish, in water, and expose for 

 a few days in the window until small Vaucheria plants are found floating on the 

 surface. Examine ver)' early in the morning and the volvox-like colonies may 

 be seen escaping from the swollen ends of the filaments. In order to observe 

 the colonies later in the morning, cover the dish, the evening before observa- 

 tion is to be made, so that the plants will be in absolute darkness until shortly 

 before the material is to be studied. Study and draw. Describe in detail the 

 formation of the compound zoospores and how they develop into new Vaucheria 



