STUDIES OF IRRITABILITY OF PLANTS PEIRCE 73 



in the shelf culture shown in Figure 3. Figure 4 shows another shelf culture, 

 but this was robbed from time to time of some of its prothalli, both arche- 

 gonial and antheridial, for purposes of examination. Pains were taken, how- 

 ever, to remove such numbers of male and female prothalli as to maintain 

 the ratio. We see that this ratio is quite different in the shelf and in the 

 turn-table cultures, there being a larger proportion of female prothalli in the 

 turn-table cultures. 



Measurements of typical archegonial prothalli from turn-table and shelf 

 cultures were very kindly made for me by a student in my laboratory, Miss 

 Viola F. Nichols, whom I wish to thank for her help. The data follow : 



PTERIS sowed X, 11, '11. SHELF TURN-TABLE 



measured IV, 15, '12. 8x4 mm. 10 x 5 mm. 



6x4 11 x 5 

 5 x 3.5 9x5 



5x3 10 x 4 



2.5 x 1 10 x 5 

 4 x 2.5 8x4 



4x3 8 x 4.5 



average' 4.85 x 2.85 mm. 9.42 x 4.64 mm. 



The difference in size, so apparent to the eye, is thus confirmed by meas- 

 urements. Accompanying this difference in size is a more than correspond- 

 ing difference in the number of archegonia and antheridia on the turn-table 

 prothalli as compared with those in the shelf cultures. This ratio is nearer 

 four or five to one. For this figure I am also indebted to Miss Nichols, but 

 my own observations correspond. 



These figures, together with an inspection of the photographs, furnish 

 the evidence of differences between Pteris prothalli grown on turn-tables in 

 approximately uniform illumination on all sides successively, and others 

 grown on the shelf with the light always from one side. These differences 

 are of three main sorts: 1st, in size of the vegetative parts, the prothalli; 

 2nd, the numbers of reproductive organs; 3rd, the proportions of male and 



