APPENDIX 237 



resting stage in which the cells develop heavy walls and the contents 

 become reddish. Such stones may be dried, and when placed again in 

 rain water will develop a new generation of green motile cells. Cultures 

 may thus be carried along from season to season. The oospores of 

 Volvox will fall to the bottom of an aquarium among the sediment and 

 after a time produce a new generation of Volvox colonies. 



H. Hydrodictyon. Hydrodictyon may be readily grown in aquaria, 

 where it will form successive generations of nets. Material cultivated 

 in diluted Knop's solution, one half to one per cent (Sec. 200, A), is 

 likely to form zoospores after a few hours if transferred to pond or tap 

 water. Nets placed in a five per cent solution of cane sugar (Sec. 200, C) 

 are likely to produce gametes in a few days. 



12. Coleocheete. Coleochcete frequently appears in aquaria, forming 

 green disks on the sides of the glass. These may readily be detached 

 with a scalpel and gathered with a pipette, and show the vegetative 

 structure of the plant excellently. We have never known the alga to 

 produce sexual fruit in aquaria. 



13. Diatoms. The shells of diatoms in polishing powders and earths, 

 or masses of fresh material, may be cleaned as follows. Heat in a small 

 porcelain evaporator with c.p. sulphuric acid and slowly drop in a few 

 crystals of sodium nitrate. After cooling rinse the sediment thoroughly 

 in water, decanting frequently. It may then be dried on a slide and 

 mounted permanently in balsam. 



14. Bacteria. It is much more important that the bacteria be studied 

 for the appearance of the growths en masse than that detailed micro- 

 scopical examinations be made of these minute organisms. The micro- 

 scopical examination of the " fur " from the teeth gives one of the most 

 striking assemblages of forms. The cultures are best made by groups of 

 students working with a set of Petri dishes in common. Excellent cul- 

 tures of bacteria may^ be obtained in Petri dishes on potato agar as a 

 substratum (Sec. 203), and these are preferable to cultures on slices of 

 potato, but the preparation of agar requires more time and considerable 

 laboratory equipment. 



15. The bread mold. Individual cultures of bread mold may be 

 readily made by the students and are more convenient for study than a 

 large culture in common. Place small pieces of bread in watch glasses 

 and set the latter over wet blotting paper in saucers, covering each with 

 a tumbler. To make sure of good growths inoculate the pieces of bread 



