440 MODIFIED CIECUMXUTATION. CHAP. 



branches which had been fastened in various positions 

 or turned upside down. 



In our observations on the cotyledons of seedling 

 plants, we often felt surprise at their persistent hori- 

 zontal position during the day, and were convinced 

 before we had read Frank's essay, that some special 

 explanation was necessary. De Yries has shown* 

 that the more or less horizontal position of leaves is 

 in most cases influenced by epinasty, by their own 

 weight, and by apogeotropism. A young cotyledon 

 or leaf after bursting free is brought down into its 

 proper position, as already remarked, by epinasty, 

 which, according to De Yries, long continues to act 

 on the midribs and petioles. Weight can hardly be 

 influential in the case of cotyledons, except in a few 

 cases presently to be mentioned, but must be so with 

 large and thick leaves. With respect to apogeotropism, 

 De Vries maintains that it generally comes into play, 

 and of this fact we shall presently advance some 

 indirect evidence. But over these and other constant 

 forces we believe that there is in many cases, but we 

 do not say in all, a preponderant tendency in leaves 

 and cotyledons to place themselves more or less trans- 

 versely with respect to the light. 



In the cases above alluded to of seedlings exposed 

 to a lateral light with their hypocotyls secured, it is 

 impossible that epinasty, weight and apogeotropism, 

 either in opposition or combined, can be the cause of 

 the rising of one cotyledon, and of the sinking of the 

 other, since the forces in question act equally on both ; 

 and since epinasty, weight and apogeotropism all act 

 in a vertical plane, they cannot cause the twisting of 

 the petioles, which occurs in seedlings under the 



* Arbeilen des Bot. Instituts in Wurzburg,' Heft. ii. 1872, pp. 

 223-277. 



