262 
METHODS IN PLANT HISTOLOGY 
good supply of antheridia 3 weeks after sowing, and the archegonia 
appear soon after, but it is well to make sowings 6 weeks before 
material is needed for use. In P. aquilina and in many others, if 
the spores are sown too thickly only antheridial plants will be ob- 
Fig. 87 .—Pteris aquilina: A, filamentous stage; B, the apical cell has been established and 
several segments have been cut off; the figure shows the initial rhizoid and also three rhizoids 
coming from the main body of the prothallium; C an older prothallium covered with antheridia in 
various stages of development; from a drawing by Miss M. E. Tarrant. 
tained (Fig. 87). If prothallia are to produce archegonia, they must 
have sufficient room and nutrition. 
The best method we have ever seen for growing fern prothallia 
was devised by Mr. M. Costello, head gardener at the University of 
Chicago. The diagrammatic Figure 88 will make the method clear. 
Select a clean flower-pot, as porous as possible, and pack it full of 
wet Sphagnum. Wet the outside of the pot and invert it in a pan of 
