42 LIVING PLANTS 



ary position, may repay closer examination. 

 A little scrutiny shows that the leaf is set 

 upon the stem in the normal manner, and the 

 vertical position attained by a quarter turn 

 near the base. Some leaves turn one v^ay, 

 and some the opposite way, to gain the de- 

 sired uprightness. 



This in itself is odd enough, but a further 

 The habit examination shows all the leaves on a plant 



of polarity to stand for the most part in one plane. If 



the plant is looked at from a certain point of 

 view, one sees the flat surfaces of the leaves, 

 partly upper and partly under surfaces, while 

 seen at right angles to the former direction 

 the leaves present their edges only. The 

 leaves of each plant, in fact, lie approximate- 

 ly in a single plane. 



Even stranger yet, the plane in which the 

 leaves lie is that of the meridian, that is, the 

 leaves of the prickly lettuce present their 

 edges north and south. The species, in fact, 

 is a so-called compass plant, and exhibits one 

 of the most curious and interesting cases of 

 physiological adaptation to be met with in 

 plants. Its polarity was first observed by 

 Dr. Stahl, professor of botany at Jena, who 

 published a very full account of the matter 

 in 1881. 



There are two species known which are pre- 

 eminently entitled to be called compass plants: 



