302 METHODS IN PLANT HISTOLOGY 



coming later from a bud near the base of the fertile shoot. The im- 

 mense strobilus, 2 or 3 inches in length, cuts easily in paraffin. 



In the other species, the strobilus is borne at the top of the vegeta- 

 tive shoot. It appears in the spring, develops, and sheds its spores the 

 same season. The younger stages may be fixed in chromic solutions, 

 imbedded in paraffin, and cut without difficulty. Later stages, if they 

 show much silica, as is likely to be the case in such species as E. 

 hiemale, had better be fixed in formalin acetic alcohol and treated with 

 hydrofluoric acid. 



The spores of Equisetum are excellent for illustrating hygroscopic 

 movements. Shake out the spores from the strobili and let them dry 

 thoroughly. They can be kept dry for years. When wanted for demon- 

 stration, put a few on a slide, moisten a little, and watch the move- 

 ments under the microsope. 



The gametophytes. — The spores of Equisetum germinate as soon as 

 they are shed, but, like all spores with a considerable amount of 

 chlorophyll, they do not long retain the power of germination. A com- 

 paratively small percentage will germinate a week after shedding, and 

 after a month, there may be no germination at all. There is no diffi- 

 culty in growing prothallia to maturity and securing stages in the em- 

 bryo, if fungi or blue-green algae do not appear and ruin the cultures. 

 Use Costello's method for fern prothallia, as described on page 316. 



Dr. Elda Walker's papers on Equisetum give directions for finding 

 prothallia in the field and raising them from spores; besides, they cor- 

 rect some current misconceptions in regard to gametophytes of Equi- 

 setum and contain a full account of the gametophyte of E. laevigatum. 



If you should not succeed with this much-condensed summary of 

 Dr. Walker's methods, consult the full papers. Boil Sphagnum 45 

 minutes, pack in boiled moist chambers to a depth of 2 inches and 

 press out surplus water. Cover until cool and then sow the spores by 

 tapping on the strobilus. Cover with glass and place in a north window. 

 If fungi appear, water with a solution of potassium permanganate, 

 putting enough crystals into distilled water to give the solution a dark 

 purple color. This does no damage and may save cultures, even after 

 fungi get in. Some algae do no damage. Cultures of Equisetum telma- 

 teia and E. kansanum grew two years, and of E. arvense, 9 months. 

 Antheridia may appear in 3 weeks; archegonia, a week or two later 

 (Fig. 95). 



In another method an ordinary greenhouse flat was filled with sifted 



