Page:Popular Science Monthly Volume 27.djvu/370

354 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 composite 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 Alvord, 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 English 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 presently return to the consideration of the general characteristics of tropical 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