72 DUDLEY MEMORIAL VOLUME 



larger prothalli bear archegonia, the smaller prothalli are antheridial. 

 Toward the upper right hand corner is a young sporophyte, showing that 

 conditions in the culture were sufficiently favorable to permit fertilization 

 and subsequent development. Microscopic examination shows that the repro- 

 ductive organs are borne on both sides of the prothallus, as uniformly as the 

 rhizoids. The prothallus has the usual cushion, which bears the rhizoids, 

 archegonia and antheridia. 



Near the lower edge of the figure is a large archegonial prothallus, 

 irregular in outline but plainly dorsi-ventral in structure. This has rhizoids 

 only on the side toward the middle of the tile. Comparing this prothallus 

 with the two nearest the center of the culture, one sees that the more uniform 

 illumination of the plants in the middle is accompanied by a more uniform 

 growth of rhizoids. The distance between the prothallus at the edge and 

 those in the middle of the tile is about 2 centimeters, but this slight difference 

 in position is accompanied by enough difference in illumination to permit the 

 plantlet near the edge to complete its usual dorsi-ventral development, while 

 the plants at the center form rhizoids and reproductive organs equally on 

 both sides. 



From this one may infer either that the fern prothallus is very sensitive 

 to light, since slight differences in illumination cause such evident differences 

 in behavior, or that it is only slightly sensitive, since exactly uniform illumina- 

 tion is necessary to overcome the usual dorsi-ventrality in any degree. 

 Whether the plant is sensitive or not sensitive can be proved only by experi- 

 ment. One of these experiments would involve constant illumination, and 

 this I hope to try shortly. 



In Figure 2 the number is larger, the distribution more regular, and the 

 results striking. 



Figure 3 is that of a shelf culture, the direction of illumination of which 

 is indicated by the arrow. The size, position, form and appendages of the 

 largest archegonial prothalli are quite as usual and normal. Rhizoids and 

 archegonia develop only on the side away from the light, the prothalli are 

 erect in response to the nearly horizontal plane along which the light is re- 

 ceived, but they are nearly flat and extend at right angles to the light and 

 they are of the usual size. The much (and normally) smaller antheridial 

 prothalli correspond in all these respects with the much more striking arche- 

 gonial. 



I do not know what may be said to be the normal ratio between arche- 

 gonial and antheridial prothalli in Pteris, and I think most botanists would 

 doubt there being any "normal" apart from the conditions or circumstances 

 in nature or in an experimental culture, but the ratio of archegonial prothalli 

 to antheridial in the two turn-table cultures figured is certainly larger than 



