STUDIES OF IRRITABILITY OF PLANTS PEIRCE 67 



sterilized tiles in crystallizing dishes, containing sterilized Knopp's Solution 

 of 0.35% concentration and covered with blackened lids. These dishes I 

 set on the turn-tables and on the shelf beside the turn-tables. I took all 

 possible pains to select branches clean and healthy-looking, but as no steriliza- 

 tion of the material was possible, it was inevitable that a certain amount of 

 moulding should take place. All of the cultures succumbed to infections 

 sooner or later, but enough of them grew well to justify a record of the experi- 

 ment, although I do not by any means regard it as concluded. 



The general characteristics of this plant are well known. Detailed de- 

 scriptions may be found in Campbell's "Mosses and Ferns" 5 and elsewhere. 

 For our purpose it is sufficient to say that the plant is dorsi-ventral to the 

 extent of having two sets of leaves : foliage leaves, which are green and closely 

 arranged, forming the upper side of the plant, and the so-called amphigastria, 

 leaves or scales not green, and overlapping along the under side of the stem. 

 The plant grows more or less closely appressed to the sub-stratum, whether 

 this is vertical, oblique, or horizontal ; that is, the plant grows at right angles 

 to the direction from which most of the light comes. If, therefore, the plant 

 be put on a horizontal surface and the light be made to fall more or less 

 horizontally upon it, the plant or its leaves should so turn that its foliage 

 leaves would stand mainly at right angles to the incident rays, and the amphi- 

 gastria should be on the side away from the light. This happens with the 

 plants growing on the shelf, receiving light always in one and the same direc- 

 tion, just as it would happen in the case of a Porella plant growing from a 

 horizontal to a more or less vertical sub-stratum out of doors. The case of 

 the plants on the turn-tables, on the other hand, is quite different, for they 

 have no darker side. The position of all the leaves and of the amphigastria 

 on the older parts of these plants changes ; they flare more from the stem. 

 On the younger, as well as on the older parts, the amphigastria become less 

 scale-like and grow more and more leaf-like. I have no doubt that this ex- 

 periment, continued with greater freedom from infections than I secured in 

 1907, would yield results entirely similar to those of Nemec 6 , but I have not 

 yet been able to repeat it and carry it through. I would suggest here only 

 that experiment seems likely to confirm the opinion of morphologists that 

 amphigastria are modified leaves, and to show that they develop as they do 

 partly because they are on the shaded parts of these plants. 



5 Campbell, D. H. The Structure and Development of Mosses and Ferns. 

 2d Edition, New York, 1905. 



6 Nemec, B. Die Induktion der Dorsiventralitat bei einigen Moosen. Bull. int. 

 de 1'Acad. Sci. de Boheme, 1904, 1906. 



