374 SUMMARY OP CURRENT RESEARCHES RELATING TO 



Lactuca Scariola, although it had beeu observed that the leaves were 

 often vertical, Stahl was the first to notice that they generally stood 

 in a meridional plane. 



In both plants, the peculiar position of the leaves is best seen 

 when they grow in unsheltered places, exposed to bright sunlight ; 

 while when crowded together, or growing in the shade, the leaves 

 generally assume the common horizontal position. The leaves of 

 Lactuca on the north side of the stem become vertical by a twisting 

 of the petiole, the upper surface of the leaf facing the east. Those 

 on the south side by a similar twisting become vertical with the 

 upper surface facing the west. The leaves on the east and west side 

 of the stem do not exhibit any torsion of the petioles, but they become 

 upright with their upper surfaces approximated to the stem. Stahl 

 took two plants growing in pots, and placed one where it would be 

 exposed to direct sunlight from 10 o'clock until 3, and kept in the 

 dark for the rest of the day ; the other was placed so that from sun- 

 rise until 10 o'clock, and from 3 o'clock until sunset, it was exposed 

 to the sunlight, but from 10 to 3 was in the dark. In the first case 

 the leaves did not assume a meridional jiosition, but in the second 

 case they did. 



That the meridional position is produced by the sun when near 

 the horizon is clearly shown by the following experiment : — A pot 

 with several young plants was placed in a window facing the north, 

 where the plants received direct sunlight a few hours after sunrise 

 and before sunset. In this experiment the leaves bent towards the 

 north with their upper surfaces turning either to the east or to the 

 west. The pot was then placed farther back in the room, so that 

 the plants were not exposed to the direct sunlight, and the leaves 

 then assumed a position at right angles to the diffused light from the 

 window. Stahl concludes that the meridional position of the leaves 

 of Lactuca Scariola is due to the common diaheliotropism observed in 

 most leaves, and that these leaves diifer from those of other plants 

 only in their greater sensitiveness to intense light. In SilpMum 

 there is a torsion of the petioles as in Lactuca; and if the petioles 

 are fastened so that they cannot bend, the blade of the leaf itself 

 twists. Stahl states that a meridional position of the leaves can be 

 seen clearly in Aplopappus ruhlginosus, and to some extent also in 

 Lactuca saligna and Chondrella juncea, and he believes that many 

 other examples will be found, especially among the plants of dry and 

 exposed regions. 



B. CRYPTOGAMIA. 



Cryptogamia Vascularia. 



Relation of Nutrition to the Distribution of the Sexual Organs 

 on the Prothallium of Ferns.* — K. Prantl has made a series of 

 experiments on the influence of different nutrient solutions on the 

 development of the sexual organs on the prothallium of ferns, especially 

 Osmunda regalis and Ceratopteris thalictroides. The following are the 

 principal results at which he has arrived: — 1. A deficiency of nitrogen 



* Bot. Ztg., ^xxix. (18S1) pp. 753-8, 770-6. 



