danger that the prothalli may be cut too late, after fertilization has 

 taken place, in which case plants of pure species would develop. 

 Since some prothalli when fully grown are barely one-eighth the size 

 of others, while many grow at varying rates of speed, it will be 

 found impossible when cutting to distinguish between the small un- 

 developed prothalli and those which though small are already fer- 

 tilized, except perhaps by the use of a powerful microscope. How- 

 ever, the newly cut archegonial sections, before being brought in con- 

 tact with antheridial sections of the other species, could be isolated 

 long enough to allow all fertilized sections to develop plants and be 

 thrown away. There is also danger that plants may develop at any- 

 time from asexual growth, such as Dr. Farlow discovered in Pteris 

 cretica. But Dr. Farlow has pointed out that in plants arising in this 

 way, the first frond springs directly from the prothallus, so that it is 

 impossible to tell where one begins and the other ends, and the first 

 root develops afterward from the base of this frond ; while in plants 

 arising from fertilized archegonia, according to Sachs, both frond 

 and root are separated from the prothallus by a slight connectile 

 known as the "foot," and the first root develops from the base of 

 this foot some time before the appearance of the first frond from its 

 apex. Thus plants arising from asexual growth, as well as from 

 self-fertilization previous to the cutting, could be detected and 

 weeded out. 



One would naturally suppose that all other plants arising from 

 these sections must necessarily be the result of cross-fertilization. 

 Unfortunately, there is doubt of that. The prothallial sections usu- 

 ally branch soon after being cut. They nearly always do so sooner 

 or later. The antheridial sections often die down for several weeks, 

 and then send up whole clusters of prothalli. Or new segments 

 may appear from any part of the margin of the old. In archegonial 

 sections a new segment frequently fills up the old sinus. Sometimes 

 the effect is that of an old prothallus with young prothalli starting 

 from the edge ; sometimes of a cluster of overlapping prothalli 

 continuous with one another at the base. Each new segment is 

 shaped like a complete prothallus, sinus and all, which naturally 

 suggests the question, may not these new segments replace the 

 organs that have been cut away ? I have not yet discovered whether 

 this occurs or not. Mr. Lowe's experience in quartering prothalli 

 would tend to show that it does not. He speaks of the branching 

 of the quarters, and states that while keeping the lower quar- 

 ters in one dish and the upper in another, no plants, with one ex- 



