318 



METHODS IN PLANT HISTOLOGY 



The heterosporous filicales. — The four genera, Pilularia, Marsilia, 

 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 laboratory type of this 

 group. It is easily grown in a pond or in an aquarium in the green- 



FiG. 107. — Osmunda cinnamomea: 

 photomicrograph of vertical section of 

 prothallium with an early stage in the de- 

 velopment of the archegonium, showing 

 the basal cell, two neck cells, and, between 

 them, the cell which is to give rise to the 

 neck canal cell, the ventral canal cell, and 

 the egg. Chromo-acetic acid; safranin, 

 gentian violet; from a preparation by Dr. 

 W. J. G. Land. Negative by Miss Ethel 

 Thomas. X425. 



Fig. 108. — Osmunda cinnamomea: 

 photomicrograph of a vertical section with 

 a young archegonium, showing the neck 

 canal cell with two nuclei, the ventral 

 canal cell, the egg, and the basal cell. 

 Chromo-acetic acid; safranin, gentian 

 violet; from a preparation by Dr. W. J. G. 

 Land. Negative by Miss Ethel Thomas. 

 X293. 



house. 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. The portions which are not under 

 water will continue to fruit during the summer and autumn. The 

 whole sporocarp cuts easily in paraffin during the development of 



