46 LIVING PLANTS 



right and a left surface; both sides function 

 alike. 



Some time ago the writer made the obser- 

 vation that the garden lettuce also show^s 

 polarity of the stem leaves, although not so 

 marked as in the wild plant. It is stronger in 

 the plain narrow leaves of the Cos and Deer- 

 tongue varieties than in the curled leaves of 

 the more common varieties. 



Both the wild and garden forms show no 

 vertical adjustments of the basal or so-called 

 root leaves, the edible part of the cultivated 

 plant. In feral plants these leaves are not 

 called upon to endure the hot sun of July and 

 August, having already performed their office 

 during spring and early summer, and died. 

 The compass plant of the prairies (Silphium), 

 on the contrary, retains its root leaves 

 throughout the torrid season. It is evident 

 that the device is primarily a midsummer ad- 

 justment, only developed in such foliar organs 

 as are destined to endure the fiercest insola- 

 tion. 



There are eight or nine species of wild let- 

 tuce indigenous to North America, but none 

 of them has yet been observed to show po- 

 larity. The species that are most at home in 

 the western prairie regions, such as Lnctaca 

 Ludoviciana, are most likely to show tendency 

 toward the habit. 



