THE WEEDS OF NEW SOUTH WALES. 125 



Prickly Lettuce or Compass Plant (Lactuca Scariola L.). 

 (COMPOSITE: Daisy Family.) 



Botanical Name. Lactuca, Latin for lettuce; Scariola, Latin for wild 

 salad. 



Botanical Description. 



Rather scabrous below, leaves suberect, radical obovate-oblong, sinuate- 

 toothed or runcinate, upper sagittate amplexicaul, auricles acute spreading, 

 branches of panicle long spreading, fruit grey. Waste places, rare, Worcester, 

 Norfolk, Essex, Kent, and Surrey; native (?) Watson; fl. July-August. Closely 

 allied to L. virosa, but prickly only towards the base; branches more erect; 

 leaves usually more runcinate ; heads smaller ; fruit narrower. (" The Students' 

 Flora of the British Islands." Sir J. D. Hooker, 2nd Ed., p. 226.) 



The Prickly Lettuce a Parent of the Common Lettuce. Alphonse De 

 Candolle (" Origin of Cultivated Plants ") says : 



Botanists are agreed in considering the cultivated lettuce as a modification of 

 the wild species called Lactwa Scariola. The common lettuce is, indeed, known 

 to botanists as L. Scariola, var. sativa. 



Vernacular Names. "Prickly Lettuce," from the prickles on the wavy 

 margins, on the midribs of the leaves, on the lower side, and lower part of 

 the stem. Its milky juice, yellow heads of flowers, and other characters, 

 show its affinity to the common vegetable lettuce. It is sometimes known 

 as " Milk Thistle." Its name of " Compass Plant " is so interesting that it 

 will be specially referred to in the next paragraph. 



The Compass Plant. In Europe the leaves of this plant markedly twist 

 themselves in the sun, so that their margins become directed upwards and 

 downwards (i.e., in a vertical position), with their margins directed north 

 and south ; hence this is called a " Compass Plant." The physiological 

 reason for this is to enable the lower and upper surfaces of the leaves to be 

 approximately uniformly presented to the action of the sun's rays. Sil- 

 phium laciniatum is another Compass Plant. The following observations 

 refer to our Silphium and Lactuca : 



Healthy living plants as they grow in the sunny meadows look as though they 

 had been laid between two gigantic sheets of paper, somewhat pressed, and 

 dried for some time in the way plants are prepared for herbariums, and had 

 then been removed from the press and set up so that the apex and profile of 

 the vertical leaf-blades point north and south, i.e., in the meridian, while their 

 surfaces face the east and west. This inclination is so well and regularly 

 observed by the living plants on the prairies that hunters are enabled TO guide 

 themselves over such regions, even under a clouded sky, by means of these 

 plants ; for this reason Silphium laciniatum has been called a " compass " 

 plant. The life of the compass plant is assisted by this placing of the vertical 

 leaves in the meridian, in that the broad surfaces are placed almost at right 

 angles to the incident sunbeams which illuminate them in the cool and relatively 

 damp morning and evening, while at the same time they are not too strongly 

 heated nor stimulated to excessive transpiration. At mid-day, on the other 

 hand, when the sun's rays only fall on the profile of the leaves, the heating 

 and transpiration are proportionately slight. It is of interest that the leaves 

 of these compass plants, as well as those of the above-mentioned lettuce, show 

 this inclination and position when they grow on level, moderately dry, unshaded 

 ground ; and that in damp and shady places, where there is no danger of over- 

 transpiration from the powerful rays of the noontide sun the twisting of the 

 leaves does not take place and they are not brought into the meridian. (Kerner 

 and Oliver, 1, 338.) 



t 64225 K 



