80 



the whole small glass capsules about i>4 inches high, with a 

 diameter of 2>^ inches, seem best suited. These hold about 26 

 c.c. of mitrient solution. This amount of nutrient solution will 

 provide sufficient nourishment for a large number of prothallia 

 to grow to maturity. However, owing to evaporation, it will be 

 necessary to add fresh solution from time to time. The advan- 

 tages of using a dish of this size are many : it is easily handled 

 by both student and teacher ; it can be placed under a compound 

 microscope and the growth of the culture observed ; it does not 

 occupy so much space on the laboratory table and therefore is not 

 so liable to accident; and, if by chance it does meet with one or 

 becomes too badly infected by algae or fungi, its loss is not so 

 great. Sometimes a culture may be freed from an algal or 

 fungous growth by lifting the prothallia with a sterilized seeker 

 or sharp-pointed scalpel from the old solution to a new one in a 

 clean dish. Care must be taken not to immerse the prothallia in 

 the solution, for if this accidentally happens it will require care 

 to make them remain on the surface afterward. Sometimes this 

 can be accomplished by drying the upper surfaces of the pro- 

 thallia with filter paper. For study by a class in beginning botany 

 it is perhaps best to select a fern whose prothallia under normal 

 conditions are monoecious. ' For this reason various species of 

 Aspidiuni or Cauiptosorus rhi::ophyllus may be used, although the 

 rarity of the latter often makes this impracticable. Among a large 

 number of the so-called " dioecious " prothallia, especially in the 

 older cultures, a large percentage of monoecious prothallia occur. 

 It is also possible by allowing, cultures of various species in which 

 large, vigorous, dioecious female prothallia predominate to be- 

 come poor in nourishment and in this way to lower their vitality 

 and thus convert them into monoecious prothallia. One way to 

 do this is not to replenish the media with new from time to time, 

 but to allow the prothallia to continue their growth on the same 

 solution upon which the spores have been sown. Since this re- 

 quires practice and skill in handling the prothallia as well as an 

 acquaintance with the prothallia of the particular species under 

 cultivation, it will hardly be practicable in most cases, unless one 



