LABORATORY STUDIES AND DEMONSTRATIONS. 215 



pear on the outside of the pots ; and as these are clean, they may 

 be removed and examined (bottom side upwards) free of all 

 dirt. 



Failing these, prothallia may almost always be found in fern- 

 houses on the tops or sides of the pots, and especially on the 

 moist earth under the benches. Care should be -taken not to 

 confound prothallia with the lighter green and relatively coarse 

 liverwort (Lunularia) often found in hothouses. 



The Sexual Organs of Prothallia. With good clean speci- 

 mens these are easily found with a rather low power. Higher 

 powers are needed to make out details. If the archegoiiia and 

 and antheridia are young they are green ; if old, brown. On 

 young prothallia antheridia only are often found, and on very 

 old ones archegonia only. 



Fertilization. This is not easy to observe, but the attempt 

 may be made by examining successively a number of very fresh 

 and vigorous prothallia in different stages. They must be 

 mounted carefully (not flooded with water), and spermatozoids 

 are generally more easily found swimming about after the speci- 

 men has been mounted a little while. 



Embryology. Except in its general features, this is too dif- 

 ficult for the beginner. He may, however, observe the later 

 stages by studying old prothallia with the young fern just ap- 

 pearing, and young ferns with the old prothallia still adherent. 



Chlorophyll and Starch. Vigorous prothallia afford excellent 

 examples of cells bearing chlorophyll-bodies in which starch is 

 easily detected. Some of the marginal cells should be examined 

 with the highest power, attention being given to the chloro- 

 phyll-bodies and their arrangement. In favorable cases one may 

 observe the opaque rod-like or oval grains inside the latter, 

 and prove by reagents that they are starch grains. 



The student should also examine, at this point, the large 

 chromatophores of Nitella, which may be obtained by pressing 

 out a drop of the contents from an internodal cell, adding dilute 

 iodine solution, and examining with a high power. In favor- 

 able cases as many as a dozen starch grains, stained blue, may be 

 found inside a single elliptical chlorophyll-body. 



