LECTURE XVII. 



Also in the Hop, Vine, Lime, various Grasses (e.g. Brachypodium 



pinnatum], Aspidium Filix-mas, Geranium sanguinettm, 



Picea excelsa, Abies pectinata. 

 Also in the following water-plants (aerial leaves), Hydrocotyle 



bonariensis, Alisma Plantago, Sagittaria, Nymphasa, Ne- 



lumbium. 



It is in the assumption of the normal fixed light-position 

 by leaves of this last description that the influence of light can 

 be most readily traced, and we will therefore devote a short 

 time to a more detailed consideration of the process. Of the 

 plants mentioned, Silphium laciniatttm and Lactuca scariola 

 are those which respond the directive influence of light in the 

 most marked manner. The leaves of these plants, namely, 

 are not only vertical when fully exposed to light, but they 

 place themselves in a vertical plane which more or less nearly 

 coincides with the meridian of the locality, whence they have 

 been spoken of as " Compass-plants." 



Stahl describes as follows the mode in which this peculiar 

 arrangement is attained. The leaves on the north and south 

 sides of the stem, respectively, undergo a torsion of 90, so that 

 their morphologically superior (dorsal) surfaces are directed 

 towards either the west or the east. The leaves borne on the 

 east and west sides of the stem either simply curve upwards, 

 so that they stand erect, or they undergo torsion such that 

 their surfaces become vertical and curve at the same time so 

 that their apices point towards either the north or the south : 

 in any case, the upper surfaces of the leaves borne on the 

 east side of the stem come to face the west, and those of the 

 leaves borne on the west side to face the east. 



The conditions which determine the assumption of the 

 vertical position by dorsiventral leaves have been investigated 

 by Stahl with reference to Lactuca scariola, and there can be 

 little doubt that the facts which he has ascertained in the case 

 of this plant are true also of the other plants which exhibit 

 the same phenomenon. He finds, in the first place, that the 

 vertical position of the surfaces is assumed only when the 

 plant is growing fully exposed in a sunny spot ; when it 

 grows in the shade the leaves are horizontal. Secondly, if 

 the plant is grown under such circumstances that it receives 



