296 



METHODS IN PLANT HISTOLOGY 



Once found, the technique is easy. Fix in the Chicago chromo- 

 acetic-osmic solution, or this sohition without the osmic acid. 



Stain in iron-alum haematoxylin and orange for the development of 

 archegonia and antheridia. Use safranin, gentian violet, orange, for 

 the development of the embryo. For the endophytic fungus which 

 stains well with the last-named combination, try the method of differ- 

 entiation of pathogen and host, described at 

 the close of the chapter on fungi. 



Selaginella. — Material of Selaginella, in 

 all phases of the life-history, is easy to 

 secure, but not so easy to handle after it is 

 obtained. As many as 340 species, mostly 

 tropical, have been described, only 3 of 

 which are common in the range of Gray's 

 Manual. Of these 3, Selagifiella apus is best 

 for sections. It is found in moist or wet 

 situations on the borders of ponds, along 

 ditches, or on moist meadows. While the 

 plant is very small, it has large spores. 

 Several of the tropical species are common 

 in greenhouses, and they fruit abundantly. 

 Vegetative structure. — Growing points and root-tips are easily cut in 

 paraffin. In most species the older parts of the stem are too hard and 

 brittle to cut in paraffin and are too small to cut well freehand. 



After fixing, treat for 2 or 3 days in 10 per cent hydrofluoric acid, 

 and then try paraffin. 



Some of the tropical species, like Selaginella wildenovii, have stems 

 nearly as large as a lead pencil, with polystelic structure, and are not 

 hard to cut. The vascular cylinder is an exarch protostele or, when 

 polystelic, each bundle is an exarch protostele. It is exceptionally 

 easy to get a brilliant, differentiated stain when once the sections 

 are cut. 



The strohilus. — Very young strobili cut easily in paraffin, but after 

 the megaspore coats begin to harden, there are few objects which 

 make more trouble than the strobili of Selaginella. For stages up to 

 the young megaspores, fix in chromo-acetic acid, with or without the 

 addition of osmic acid. For later stages, use formahn 10 c.c, acetic 

 acid 5 c.c, and 70 per cent alcohol 85 c.c. The cutting of the later 

 stages is likely to be more satisfactory if, after fixing and washing, the 



Fig. 92. — Lycopodium vohi- 

 bile: prothallium .showing crown 

 with numerous antheridia cf , and 

 archegonia 9 • X 14. 



