<a ce ea a 
PROTHALLIUM OF ONOCLEA SENSIBILIS L. 619 
the wall of the glass dish as none of the submerged prothallia 
examined were observed near the side of the dish. F ig. 5 seems 
a more normal prothallium having average sized cells. The end 
of the filament is turned so that there is a possibility of two 
growing regions here. 
An extreme case of the tendency to form branched cells is seen 
in Fic. 6. Two cells of this filament have well defined protuber- 
ances, one of which is considerably elongated. The tip of the 
apical cell is also somewhat curved. A two-armed prothallium is 
seen in Fic. 7, due to the branching of one cell. This is an unu- 
sually long filamentous prothallium. The shorter branch consists 
of two cells. Fics. 8 and 9 show great variety in the apical cell, 
each having a number of lobes. This suggests considerable di- 
versity in the subsequent growth of the prothallium. In each 
case the apical cell is curved with two lobes at the extreme tip. 
Fic. 10 shows a young stage of a two-armed prothallium. In Fic. 
II, three branches are found with their origin in one cell. Two of 
these branches are limited by cross walls. The increased activity 
of the cells where one or more branches are produced may be ex- 
plained by considering that the active cell was once the apical cell 
and in a normal prothallium would remain the center of growth. 
In sowing fern spores it is not unusual to find sporangia or a 
fragment of leaf inthe culture. It was thought that an attached 
sporangium or bit of leaf caused the sinking of some of the spores, 
thus changing their environment by completely surrounding the 
developing prothallia with water. The dish was covered with a 
loosely fitting lid so that prothallia growing on the surface of the 
water would receive a sufficient supply of oxygen. The amount of 
oxygen would necessarily be limited in the prothallia growing 
below the surface of the water. It is suggestive that the oxygen 
requirement may have been a factor in producing these irregular 
gametophytes. 
Besides the filament of elongated cells characteristic of im- 
- poverished prothallia, the accompanying drawings show that these 
prothallia differ from the normal gametophyte in one or more of 
the following characteristics: 
1. A change in the direction of growth of the filament may 
occur by the apical cell forming an acute angle with the main cell 
thread (Fic. 4). 
