346 BOTANICAL GAZETTE [MAY 
determination of every species used was made or verified by Mr. W. R. 
Maxon. Antheridial prothalli were placed in water on a slide, and 
prothalli bearing only archegonia of the species necessary to make 
the desired cross were added to these; the entrance of sperms 
into the archegonia was then followed under the microscope. After 
being left on the slide for one to four hours, the prothalli were either 
planted on soil in separate pots or were fixed and studied in section 
to see whether fusion of sperm and egg had occurred. About half 
of the prothalli in which experimental fertilization was attempted 
were planted and the other half sectioned. 
In this method there are four places where an error is especially 
likely to occur: (1) in gathering the spores of several species in one 
day, these may become mixed; (2) in planting the spores, or after 
these are sown, spores of other species may fall into the pot and con- 
taminate the culture; (3) fertilization may have taken place before 
the prothalli are removed from the pot for the experiment; (4) the 
female prothalli may bear antheridia. 
All these dangers were especially guarded against. In collecting 
spores, mature fronds were chosen, and being shaken as little as 
possible were carefully wrapped in several thicknesses of paper- 
The fruiting portions of the fronds were touched as little as possible, 
and the hands were washed or carefully wiped with a damp cloth 
between each collection. . 
The spores were sown on soil which was sterilized by heating to 
150° C. for six to twenty-four hours. All sowings were made in 
rooms in which no other ferns were ever kept. When more than one 
sowing was made on any day, these were made in widely separated 
parts of the rooms, and after each sowing all the surrounding furniture 
and walls were wiped with a wet cloth. Before and after each sowing 
the hands were carefully washed and all instruments used were steril- 
ized by boiling. The pots were kept in the laboratory where no 
other ferns were grown, or in the greenhouse. In this room of the 
greenhouse there were three fruiting specimens of ferns, but these were 
on the opposite side of the room. The cultures were covered with 
glass. In spite of these precautions, a small amount of moss 
appeared in several of the pots, and fungi occasionally made their 
entrance. It is not believed that the cultures were contaminated by 
