STUDIES OF IRRITABILITY OF PLANTS PEIRCE 71 



by them. A second sowing in other dishes was made on November 13, 1911. 

 About the same number of cultures of Gymnogramme were started on Octo- 

 ber 11 and November 13. The results were similar to those obtained with 

 Pteris but less striking, because of the smaller size of the prothalli. Further- 

 more, the Pteris material was so much freer from contaminating blue-green 

 algae that the cultures were correspondingly more satisfactory. Since steriliz- 

 ing the spores is impossible, it is not usually possible to make a pure culture 

 of fern prothalli directly from the spore. 



The spores of both ferns germinated well, behaving during the earlier 

 stages on the turn-tables and on the shelf as before described 11 . In the ex- 

 periments previously reported, the spores of Gymnogramme germinating on 

 turn-tables developed into thin prothalli, crumpled or waved instead of flat, 

 heart-shaped instead of conical or cylindrical, but with rhizoids, antheridia 

 'and archegonia in equal numbers on both sides, and with the prothalli stand- 

 ing erect, but at all possible angles on the tiles. In the shelf cultures, on the 

 other hand, the prothalli were normal in shape and almost linearly placed, in 

 ranks surprisingly regular, at right angles to the incoming light from the 

 window. These results I thought might be due to the slow revolution of my 

 turn-tables and I hoped by using quicker ones I might obtain cylindrical 

 or conical prothalli. I have not yet. Why, I do not know. It may be that 

 the turn-tables, revolving four times a minute instead of once in fifteen as 

 before, are still too slow, or it may be that continuous as well as uniform 

 illumination is necessary. 



The results of five months' growth on turn-tables are shown in Figures 

 1 and 2. Figures 3 and 4 show shelf cultures. These two sets of photo- 

 graphs of Pteris cultures sowed on October 11, 1911, were kindly taken for 

 me by Mr. James McMurphy, Instructor in Botany in this University. The 

 culture dishes were uncovered and placed on the horizontal stand of a verti- 

 cally working camera, so focused as to give a picture double natural size.* 

 The magnification was the same for all five photographs. This was made 

 possible by the uniform thickness of the tiles and of the glass of the dishes 

 The four figures are, therefore, perfectly comparable. 



Figure 1 shows, within the circular line which indicates the bottom 

 of the crystallizing dish, and upon a square porous tile, the corners of which 

 have been knocked off to fit the glass dish, a large number of fern prothalli 

 of two very different sizes. These prothalli are erect or nearly so. Those 

 nearest the center of the tile (the center of revolution) are most nearly erect 

 and most plainly show the copious growth of rhizoids on both sides. The 



Annals of Botany, XX, p. 454+, 1906. 

 * Reduced to natural size in the figures. 



