12 



PLANT RELATIONS. 



FIG. 5. The common prickly lettuce (Lactuca 

 Scariold), 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 leli- 

 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 



