12 



PLANT STUDIES 



^''iG. 5. The cmniii.m prickly lettuce [Laciuca 

 ■Srt(rioltt), showing the leaves standiiif:; edge- 

 wiHc, and in a general north and south plane. 

 —After Arthur and MacDougal. 



must not be siij^posod 

 thtit there is any ac- 

 curacy in tlie north or 

 sou til direction, as the 

 edgewise position 

 seems to be tlie signifi- 

 cant one. In tlie ros- 

 in-weed probaljly the 

 north and soutli direc- 

 tion is tlie prevailing 

 one ; but in the prickly 

 lettuce, a very eommon 

 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 

 Pig. 6). 



l(i. Heliotropism. — 

 The influence of light 

 upon the positions of 

 leaves and otlier or- 

 gans is known as lieli- 

 otro])ism, and it is one 

 of the most important 

 of those external influ- 

 ences to which plant 

 organs respond (see 

 Figs. 0. 43). 



It should be under- 

 stood clearly tliat this 

 is but a slight glimpse 



