762 Botanical Microtechnique 



that are not locally axailable can be purchased from supply houses, 

 preserved either for bulk specimens or for sectioning, as specified by 

 the purchaser. 



These fragile plants must be collected and handled ^vith care, 

 taking precautions to keep the plants moist and undamaged until the 

 time of killing. Entire plants preserved in fluid are indispensable for 

 teaching. Fhe most useful bulk preservatives are described in Chap. 

 10. Preservation and processing for embedding must be carried out 

 with j^ainstaking care, approaching cytological methods. 



Hepaticae 



The following recommendations, based on Marchantia, ^\■ill apply 

 to a wide range of liverworts. 1 he young, actively growing thallus is 

 usually sectioned to show the construction of the pores, the highly 

 spongy chlorenchyma, and the gemmae. Cut out 4-mm. squares of 

 tissue. Genmia cups should be excised with a small scjuare of thallus. 

 Antheridial and archegonial receptacles should be collected when 

 they are just beginning to be elevated above the thallus. The game- 

 tangia are at their best at this stage. When the archegonial disk has 

 been fully elevated, make a collection lor the developing sjK)roph}tes. 

 A complete series of developmental stages may be obtained by collect- 

 ing at intervals. Kill in weak chrome-acetic or Craf I. A closely graded 

 alcohol-xylene series is recommended. 



The thickness of sections can be judged best at the time of 

 microtoming. Examine a few trial sections by melting the ribbon on 

 a slide, and decide whether the trial thickness includes the desired 

 structmes and is sulficiently thin to show internal detail. Sections 

 will range from (i ^i for a careful examination of young antheridia, to 

 15 [.I for maturing capsules. A simple hemalum stain, with perhaps 

 a light (ouiUerstain of erythrosin, sets oil all essential structures very 

 well. A nutltifolor slain combination is cpiite pointless. Iron hema- 

 (ox\lin is the uhimaie dioice for cytological details. 



Gennnae can be studied conveniently by dissecting them from 

 genmia cups of fresli or preserved thalH. Permanent \\]i()le-iH()unt 

 slides of gennnae are of little value, but such mounts can be made 

 by the methods outlined for ])repaiing filamentous green algae. 

 Microtome sections aie ne(essai\ to show the initiation and deMloj)- 

 ment of genmiae. 



Riccia and AnI/kx cro.s are somewhat moie dilficult than the 

 foregoing type, because the sex organs are sunken in the thallus. 

 Skillful freehand sectioning reveals the presence of sex organs and 



