240 



Methods in Plant Histology 



for about an hour; then transfer to the chromo-acetic acid without 

 any osmic. If the gradual processes of dehydrating, clearing, and 

 infiltrating have been carefully observed, about 15 to 20 minutes in 

 the bath should be sufficient (Figs. 80, 81, 82). 



For the development of the antheridium and sperm, and especially 

 for the blepharoplasts and their transformation, cut about 3 fj, in 

 thickness and use iron-haematoxylin; for the development of arche- 

 gonia, cut at 5 ^ and stain in the safranin, gentian-violet, orange 

 combination. 



FIG. 80. Osmunda cinnamomea: photomicrograph of vertical section from the 

 notch toward the base of the prothallium showing four stages in the development of 

 the archegoniurn chrome-acetic acid; safranin, gentian-violet; from a preparation by 

 Dr. W. J. G. Land. X425. 



The Heterosporous Filicales.- -The four genera Pilularia, Mar- 

 silia, Salvinia, and Azolla are aquatic, the first two growing rooted 

 but more or less submerged, and the other two floating freely on 

 the water. Marsilia is the most available and convenient labora- 

 tory type of this group. It is easily grown in a pond or in an 

 aquarium in the greenhouse. In setting it out in a pond, select a 

 place with a gently sloping bank, so that part of the material may be 

 under water and part may creep up the bank. In the greenhouse, a 

 rectangular aquarium may be tilted to secure the same conditions. 



