STUDIES OF IRRITABILITY OF PLANTS PEIRCE 65 



furnished a fairly smooth substratum of convenient extent. Two considera- 

 tions prevented, namely : the solubility of Plaster of Paris, and its color. The 

 latter could have been modified by lamp-black or any other insoluble pigment, 

 but the constant presence of an undue amount of calcium sulphate in the 

 water which the plants were to absorb seemed to me unnecessary and possibly 

 confusing. Flower-pot tile is certainly more nearly like soil in color and 

 composition than is Plaster of Paris, more convenient to handle, and readily 

 enough obtained in any desired size, if a reasonable number of tiles be ordered 

 at any one time. 



The spores are sowed as uniformly as possible on the now damp tile, 

 which is standing in sterilized Knopp's Solution in the dishes. The spores 

 are sowed from stiff smooth writing paper by tapping the paper with a pencil 

 or paper knife in such a way as to discharge a fairly, even shower of spores 

 upon the tiles. The culture dishes are then marked and put in place on the 

 turn-tables, and on the shelf beside them as controls, respectively. 



The speed of the turn-tables is a matter of considerable importance. 

 The greater the speed, the greater the amount of power required. In no case, 

 however, have I used a speed at which the centrifugal force, even at the edge 

 of a culture, could have had any part in the result ; and at the center of revo- 

 lution, which is also the point of most nearly equal illumination, there would 

 be no centrifugal force. The speeds which I have so far used, in addition 

 to those previously reported*, are the following: 



10 turn-tables making four revolutions a minute. 

 20 " " two " " " 

 10 " " one 



I have also arranged to have a fifth row of ten turn-tables, so geared as 

 to make a complete revolution in two minutes. I may add that, although 

 I have so far used the turn-tables only on vertical axes, I have, nevertheless, 

 had the shelves so attached to the frame of the shelving in the window that 

 they may be set at any desired angle between the vertical and the horizontal. 

 The positions and structures of the gears of the vertical distributing shafts 

 and of the first turn-tables in each row must be and may be modified accord- 

 ingly. Obviously, if the turn-tables are to be used in a position in which 

 their axes would point obliquely downward, it would be necessary to use cups, 

 into which the axes could be locked; but for my experiments so far, no modi- 

 fication of the cups has been necessary. The cups carrying the axes of the 

 turn-tables are cast steel with a steel ball of suitable size in the bottom of 



4 Annals of Botany, XX, 1906. 



