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METHODS IN PLANT HISTOLOGY 



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- 

 tained (Fig. 103). If crowded, fern prothallia often have peculiar shapes 

 and produce antheridia very early (Fig. 104). If they are to produce 

 archegonia, they must have sufficient room and nutrition. 



Fig. 103. — 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. 



The best method we have ever seen for growing fern prothallia was 

 devised by M. J. Costello, head gardener at the University of Chica- 

 go. The diagrammatic Figure 105 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 water. 

 Sow the spores on the surface of the pot and cover with a bell jar. No 



