Brown: Apogamy in Phegopteris polypodioides 21 



In the case of the cultures which were kept in the greenhouse, 

 the majority of the prothallia growing on the modified Prantl's 

 and Knop's solutions did not develop beyond a filamentous 

 stage and lived only from five to six weeks. This behavior was 

 attributed to temperature rather than light: the temperature 

 in the greenhouse being very high during the greater part of 

 the day, the culture media became overheated, with the result 

 that the prothallia, lacking in vigor on account of an insufficient 

 supply of some essential element in trie nutrient solution, were 

 unable to withstand the unfavorable conditions of temperature. 

 A few prothallia in the greenhouse cultures on Prantl's and 

 Knop's unmodified solutions survived, however, and developed 

 into normal heart-shaped thalli. 



The length attained by the prothallium during its filamentous 

 stage varied greatly, but even in cultures on the full nutrient 

 solutions it was not uncommon to find prothallia with filaments 

 of seven and eight cells in length. These usually occurred in 

 crowded regions of the cultures. In the cultures on the modified 

 solutions the length of the filaments was even greater, some having 

 as many as twelve to fifteen cells. The length of the cells also 

 varied, some being long and others short, but with the long 

 cells predominating. It is customary for the length of the 

 filament of the prothallia of any species of ferns to vary under 

 unfavorable environmental conditions, but the range of varia- 

 bility in length seems fixed for each species. In this respect 

 Phegopteris polypodioides showed the greatest range of varia- 

 bility of any prothallia of the Polypodiaceae that have come 

 under the author's observation. 



Branching was exceedingly varied and occurred frequently in 

 the cultures on the modified solutions, while on the unmodified 

 solutions it was not uncommon. Many times the first cell of 

 the filament, instead of dividing transversely, divided length- 

 wise, giving rise to two distinct filaments, which later broadened 

 into prothallia. Sometimes other cells of the filaments divided, 

 giving rise to branches which in turn broadened into prothallia. 

 In other cases a filament of three cells was first formed, and then 

 the first and second cells of the filament divided lengthwise, 

 giving rise to side branches. These side branches continued 

 their growth as filaments for a time, and then branched dichoto- 

 mously. In still other cases the end cell of a filament of four 



