216 CULTURE METHODS 



THE CULTUEE OF EEENS 



208. The culture of fern prothallia. Fern prothallia may be cultivated even 

 more easily than moss protonemata. The method is essentially the same. 

 The spores of common greenhouse ferns will germinate readily, but the pro- 

 thallia of some present abnormalities due to apogamy (Principles, Sec. 311), 

 so it is better to sow the spores of some of the wild ferns, such as Pteris 

 aquilina, species of Onodea, Aspidium, Polypodium, etc. Such spores gen- 

 erally retain their vitality for a year or more. The spores of Osmunda, on 

 the contrary, and sllso those of Equisetum live only a few days and must be 

 sown at once at maturity, but then give very luxuriant cultures. 



The spores, crushed out of their sporangia on a piece of paper, are blown 

 over the surface of earth in bulb pots or shallow dishes. These are covered 

 with glass and the pot is set in a saucer and watered from below. The earth 

 in the pot may be sterilized with advantage (Sec. 206). The culture should 

 not be kept too moist, for there is danger of the prothallia damping off. 

 Prothallia will begin to develop antheridia in three or four weeks and will 

 be full-grown in six weeks. Care should be taken not to sow the spores too 

 thickly, at least in portions of the dish, for crowded growths of prothallia 

 remain dwarf and only develop antheridia. 



Growths of young fern sporophytes are obtained by flooding a culture of 

 mature fern prothallia for an hour or more, closing the bottom of the pot tem- 

 porarily as described in See. 207. In a few weeks the first leaves of the young 

 fern plants will appear, growing up in the notch of the large fern prothallia. 



809. Water ferns. The floating water ferns Salvinia and Azolla, like the 

 floating liverworts (Sec. 205), will grow in glass vessels if they have pure air 

 and plenty of water, but they do much better in large tanks or cement basins 

 in greenhouses. Marsilia is hardy and grows well in tanks or basins. Sal- 

 vinia and Azolla may be kept thus over winter, and in the spring, when 

 placed in ponds out of doors, will generally do well. Marsilia is easily intro- 

 duced into ponds, where it forms thick growths in shallow water. Salvinia 

 is not uncommon under cultivation in water-lily ponds of city parks. 



THE CULTUEE OE SEED PLANTS 



210. Directions for the culture of seed plants. It is hardly worth while to 

 give an account of the manner in which such seed plants as are needed for 

 studies and demonstrations are best cultivated. The advice of a competent 

 florist will be found more helpful than any set of printed directions. Das 

 Pflanzenmaterial fiir den botanischen TJnterricht, by Dr. P. Esser, I. Tell, 

 Cologne, 1903, gives much useful information. Its price is Marks 3.20. 



