176 



BOTANY 



the filaments branch ? Are they fastened at one end ? A filament is com- 

 posed of a number of cells placed end to end. Notice a single cell rather 

 carefully with the low power first, and then under the high power. The 

 following structures can be made out by the careful observer : — 



(a) The cell wall. 



(b) The chlorophyll bands. Determine if there is one or more in a 

 single cell. The number varies in different species. 



(c) The colorless protoplasm ; this is usually best seen close to the cell 

 wall. 



(d) The nucleus, suspended in the middle of the cell by strands of proto- 

 plasm. 



Draw a single cell as you see it under the high power of the microscope, 

 and label each part mentioned above. 



Pond scum may grow by a simple division of the cells in a 

 filament. Another method of reproduction is seen in this plant. 

 The cells of two adjoining filaments may push out tubes which 



meet, thus connecting the cells of two different 

 filaments with each other. Meantime the proto- 

 plasm of the cells thus joined condenses into two 

 tiny spheres; the bands of chlorophyll are broken 

 down, and ultimately the contents of one of the 

 cells passes over tho tube and mingles with the cell 

 of the neighboring filament, with which it was pre- 

 viously connected by the tube formed from the 

 cell walls. As in black mold, the result of this 

 process of fusion is a thick-walled resting cell which 

 we call a zygospore. This cell can withstand con- 

 siderable extremes of heat and cold, and may be 

 dried to such an extent that it is found in dust or 

 C^ in the air. Under favorable conditions, this spore 



' ' ' will germinate and produce a filament. 



Conjugation. — The process in which two cells of 

 equal size unite to form a single cell is called conju- 

 gation. It is believed to be a sexual process which 

 corresponds in a way to the fertilization in the 

 If material is obtainable, draw several stages in 



Conjugation of 

 Spirogyra; zs, 

 zygospore; /, 

 fusion in prog- 

 ress. 



higher plants. 



the process of conjugation.^ 



^ Material which shows conjugation is not always easy to obtain. Conjugation 

 usually takes place most freely in the fall of the year. When material is obtained 

 it may be preserved in a 4 per cent solution of formol. Material killed in a 5 per 

 cent solution of chromic acid and then preserved in 70 per cent alcohol or 4 per cent 

 formol shows the details of cellular structure. 



