PTERIDOPHYTES— FILICALES 



317 



would be better to say, harder to recognize, for it is also subterranean 

 and tuberous and, besides, looks so much like the root that you may 

 not recognize it, even when you have it in your hand. 



The embryo. — Instructive mounts of the whole embryo, with the 

 prothallium still attached, can be made by the Venetian turpentine 

 method. Iron-alum haematoxylin, with the stain not too deep, is good; 

 phloxine and anilin blue are more transparent and will show the struc- 

 ture of the root, if the two stains are well balanced. 



Fig. 106. — Osmunda cinnamomea: photomicrograph of vertical section from the notch toward 

 the base of the prothallium showing four stages in the development of the archegonium. Chromo- 

 acetic acid; safranin, gentian violet; from a preparation by Dr. W. J. G. Land. Negative by Miss 

 Ethel Thomas. X425. 



For sections, cut longitudinally, perpendicular to the prothallium. 

 Pteris longifolia may show the young embryo within 3 or 4 weeks; 

 Osrnunda, somewhat later. 



Nephrodium molle, often found in greenhouses, is generallj^ par- 

 thenogenetic, the embryos developing from vegetative cells of the 

 prothallium. The prothallia are very small, but they can be piled up 

 Hke little sheets of paper. It is better to dehydrate in a Petri dish, 

 piling the prothallia into little groups and weighting them down a 

 little with a thick cover glass or very thin slide. A whole group can 

 then be cut at one time, yielding many stages with comparatively 

 little labor. Pteris cretica, an omnipresent greenhouse form, is also 

 very often parthenogenetic. 



