Pteridophytes Filicales 237 



The various relations of sorus and indusium are best illustrated 

 by rather thick sections (10 to 20 /*) of material in which the oldest 

 sporangia have barely reached the spore stage. Fix in formalin 

 alcohol and stain in safranin and anilin blue. 



For the development of sporangia, use Flemming's weaker solu- 

 tion. The sections should be 5 to 10 p in thickness. For the reduc- 

 tion of chromosomes, the sections should not be thicker than 3 to 5 JJL. 

 Osmunda is particularly good for this purpose because the number of 

 chromosomes is comparatively small. The young sporangia of 

 Osmunda cinnamomea and 0. Claytoniana show the mother-cell stage 

 in the autuirn, but the division into spores does not occur until the 

 following spring, in the vicinity of Chicago, the mitotic figures being- 

 found during the latter part of April (Fig. 78). 0. regalis does not 

 reach the mother-cell stage in the autumn. Material for mitosis 

 should be collected during the first two weeks in May. Various 

 species of Pteris are common in greenhouses and are very good for 

 development of sporangia. Any fern of the Aspidium type will yield 

 a good series, and some, like Cyrtomium, may show a fine series in a 

 single sorus. Marattia, which is likely to be found in botanical 

 gardens, will illustrate the "synangium' : type; Angiopteris has a 

 sporangium which forms an easy transition to that of the Cycadales. 



The Prothallia. Prothallia can usually be found on the pots in 

 the ferneries of greenhouses. Ripe spores of some fern or other can 

 be obtained at any greenhouse at any time in the year, and spores 

 of most of our native ferns germinate well and produce good pro- 

 thallia, even if the sowing is not made for several months after the 

 spores have been gathered. 



Fine prothallia of Pteris aquilina have been grown two years 

 after the spores were gathered. Some, however, must be sown at 

 once, or they will not germinate at all. Spores which are large and 

 contain enough chlorophyll to make them appear greenish should 

 be sown at once. The spores of the common Osmunda regalis, and 

 of the other members of the genus, must be sown as soon as ripe, or 

 they fail to germinate. The prothallia of 0. regalis, if carefully 

 covered with glass, may be kept for a long time, and they become 

 quite large. Prothallia of this fern in the writer's laboratory pro- 



