PLANT STUDIES 
FIG. 5. The common prickly lettuce (Lactuca 
Scariola), showing the leaves standing edge- 
wise, and in a general north and south plane. 
After ARTHUR and MACDOUGAL. 
must not be supposed 
that there is any ac- 
curacy in the north or 
south direction, as the 
edgewise position 
seems to be the signifi- 
cant one. In the ros- 
in-weed probably the 
north and south direc- 
tion is the prevailing 
one ; but in the prickly 
lettuce, a very common 
weed of waste grounds, 
and one of the most 
striking of the compass 
plants, the edgewise 
position is frequently 
assumed without any 
special reference to the 
north or south direc- 
tion of the apex (see 
Fig. 5). 
16. Heliotropism. 
The influence of light 
upon the positions of 
leaves and other or- 
gans is known as lieli- 
otropism, and it is one 
of the most important 
of those external influ- 
ences to which plant 
organs respond (see 
Figs. 6, 43). 
It should be under- 
stood clearly that this 
is but a slight glimpse 
