STUDIES OF IRRITABILITY OF PLANTS PEIRCE 69 



cease to be symmetrical, the side of the cone away from the light growing out 

 in a form more or less shelf-like. This is the beginning of the thallus of the 

 more or less mature form. 



In cultures I have never carried the plants beyond this beginning six 

 months or thereabouts after sowing the spores for although plants of the 

 same species normally survive the summer out of doors 9 , they do not with- 

 stand the much more complete drying in a culture dish, or, if an attempt is 

 made to keep them moist over summer, they succumb to fungus enemies. I 

 do not know what would happen if the cultures were kept continuously on the 

 shelves and on the revolving turn-tables, for it has never been possible for me 

 to stay in my laboratory throughout the long summer vacation, and I have 

 so far been unable to arrange to have my clock-work regularly wound, i. e., 

 daily throughout my absences. It is usually easier, in a laboratory as well 

 as elsewhere, to provide apparatus than to secure assistants. Hence, at the 

 end of the college year, in May, I am obliged to take my cultures from their 

 places on shelf and turn-table and set them away in a dark cupboard. They 

 remain there till September. During these months they have succumbed to 

 mould or drought. Some day, however, I shall be able to carry them along 

 continuously from the spore to the production of spores again. 



Turning now once more to the plantlets subjected, on the turn-tables, 

 to light from all directions successively, we find that they maintain the erect 

 position which they assume immediately on germination. They thicken at 

 the ends away from the spores, and, since they are revolved in a horizontal 

 plane and receive light mainly horizontally, they become vertical. I do not 

 think the force of gravity, or the presence of water below them, or any othei 

 influence than light has much to do with the erect position of these plantlets. 

 They become erect cones standing on their apices, and, uniformly on all 

 sides, they develop rhizoids, which attach them to the tiles. The little plants 

 are thus stayed and kept from toppling over. They keep pace in their growth 

 with the plants receiving light from one side only, on the shelf, and after a 

 time exceed them considerably in size. On account of the difficulties pre- 

 viously enumerated, however, I have never been able to carry these plants 

 through the summer or continue the experiment for more than seven months. 

 Though the plants on the shelf are at first and for some weeks radial in 

 structure, they sooner or later go over to the dorsi-ventral form under the 

 influence of light falling upon them from one direction only. This form 

 they maintain throughout all but the very early stages of their existence. 

 The plants revolving on the turn-tables, on the other hand, remain radial in 



9 Campbell, D. H. Resistance of drought by liverworts. Torreya, IV, 1904. 



