388 ORGANIC GROWTH 



The parenchyma of the leaves of plants which grow in the 

 light differs completely in cell-structure from that of those 

 which grow in the shade : the cells in the two cases are of 

 quite different shape, in the latter long and prismatic, in 

 the former short, and so on. 1 



The so-called compass-plants place their leaves in sun- 

 shine so that their edges are directed to the north and south, 

 whereby the leaf is least exposed to the sun. This peculiarity 

 was first observed in the North American Silphium laciniatum, 

 but is found equally developed, according to E. Stahl, in a 

 native species of lettuce, Lactuca scariola. 2 The vertical 

 leaves turn their largest surface to the rising sun. In pro- 

 portion as the sun rises higher, the angle at which its rays 

 strike the leaves becomes smaller, until finally at mid-day all 

 the leaves when regarded in the direction of the sun's rays 

 present only their edges to the eye. In the afternoon the 

 incident angle of the sun's rays upon the leaves again 

 gradually increases, so that towards evening the light again 

 strikes them at right angles. Silphium laciniatum belongs 

 like Lactuca to the Compositse. But the leaves of many 

 Papilionacese, e.g. of the beans, in strong sunshine place 

 themselves edgewise : by the twisting of the articular pro- 

 tuberance the leaves are brought into the position which 

 presents the smallest surface to the sun. By this means 

 excessive heating and illumination are avoided. 



Plants show this property especially in dry regions, and 

 it is probably much more widely distributed than it has yet 

 been observed to be. However, some other plants are known 

 in which it occurs in a less degree. 



This, therefore, is a case in which the action of stimulation 

 is useful, as it is for instance in the carnivorous plants, in 



1 Cf. E. Stall, Ueber den Einfiuss des sonnigen oder schattigen Standorts 

 auf die Ausbildung der Laubblatter, Jenaische Zeitschr. Bd. xvi. 1883. 



2 E. Stahl, Ueber sogenannte Compasspflanzen, Jenaische Zeitschr. Bd. xv. 

 1881. 



