354 THE POPULAR SCIENCE MONTHLY. 



of the beech, elm, sycamore, or, in fact, of most of our trees, this would 

 involve a certain amount of waste ; but in the black poplar, as Fig. 15 

 shows, the leaves when hung in their natural position are quite detached 

 from one another. 



Another interesting case of a species with vertical leaves is the 

 prickly lettuce {Lactuca scariola), while those of L. muralis and L. 

 virosa are horizontal. With this position of the leaves is connected 

 another peculiarity, especially well marked in the so-called " compass " 

 plant of the American prairies (Silphium laciniatum), a yellow com- 

 posite not unlike a small sunflower, which is thus named because the 

 leaves turn their edges north and south. This has long been familiar 

 to the hunters of the prairies, but was first mentioned by General Al- 

 vord, who called Longfellow's attention to it, and thus inspired the 

 lines in " Evangeline " : 



" Look at this delicate plant, that lifts its head from the meadow, 

 See how its leaves are turned north, as true as the magnet; 

 This is the compass-flower, that the finger of God has planted 

 Here in the houseless wild to direct the traveler's journey 

 Over the sea-like, pathless, limitless waste of the desert." 



The advantage of this position, and consequently the probable reason 

 for its adoption, is that in consequence of it the two faces of the leaf 

 are about equally illuminated by the sun ; and in connection with this 

 we find that the structure of the leaf is unusual in two respects. The 

 stomata are about equally abundant on both surfaces, while pallisade- 

 cells, which are generally characteristic of the upper surface, are in 

 this species found on the lower one also. 



The leaves of the Lactuca scariola have also, when growing in 

 sunny situations, a tendency to point north and south. Under such 

 circumstances also they have a layer of pallisade-cells on each side. 



Hitherto I have dealt with plants in which one main consideration 

 appears to be the securing as much light and air as possible. Our Eng- 

 lish trees may be said as a general rule to be glad of as much sun as 

 they can get. But a glance at any shrubbery is sufficient to show that 

 we can not explain all leaves in this manner, and in tropical countries 

 some plants at any rate find the sun too much for them. I will pres- 

 ently return to the consideration of the general characteristics of tropi- 

 cal vegetation. In illustration, however, of the present point, perhaps 

 the clearest evidence is afforded by some Australian species, especially 

 the eucalypti and acacias. Here the adaptations which we meet with 

 are directed, not to the courting, but to the avoidance, of light. 



The typical leaves of acacias are pinnate, with a number of leaflets. 

 On the other hand, many of the Australian acacias have leaves (or, to 

 speak more correctly, phyllodes) more or less elongated or willow-like. 

 But if we raise them from seed we find, for instance, in Acacia salicina, 

 so called from its resemblance to a willow, that the first leaves are pin- 



