266 
METHODS IN PLANT HISTOLOGY 
Nepkr odium is generally parthenogenetic. The embryos, devel¬ 
oped while the prothallia are still very small, come from tissue cells of 
the prothallium. Pteris cretica is frequently parthenogenetic. 
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 laboratory 
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 rec¬ 
tangular 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 sporangia, the division of the spore 
mother-cells, and even during the earlier stages in the formation of 
spores. Except in the case of the youngest sporocarps, it is better 
to cut off a small portion at the top and at the bottom to facilitate 
fixing and infiltration. The mother-cell stage and the young spores 
will be found in sporocarps which are just beginning to turn brown. 
In nature, no further nuclear divisions take place within the sporan¬ 
gium until the next spring, but the wall of the sporocarp becomes 
extremely hard. Sporocarps for germinating should not be collected 
until they are so hard that it is impossible to crush them between the 
thumb and finger. They can be kept in a box until needed for use. 
When you find them in good condition, make a big collection, for 
they retain their power of germination almost indefinitely, sporocarps 
from poisoned herbarium specimens 50 years old germinating readily. 
Sporocarps which have been kept in 95 per cent alcohol for years ger¬ 
minate almost as quickly as those which have been kept in a dry box. 
To germinate sporocarps, cut away a portion of the hard wall along 
the front edge and place the sporocarp in a dish of water. The 
gelatinous ring with its sori will sometimes come out in a few minutes. 
In less than 24 hours, sometimes within 10 or 12 hours, microspores, 
starting from the one-cell stage, will produce the mature sperms; 
and the development of the female gametophyte is equally rapid. 
Starting with the uninucleate megaspore, the stage found when the 
gelatinous ring comes out, the archegonium may be developed and 
fertilization may occur within 12 hours; and within 36 hours, stages 
