Page:Popular Science Monthly Volume 19.djvu/109

Rh sometimes rage in the Campagna. The other great enemy of the tree is cold, and this offers an almost insurmountable obstacle to its successful culture in Great Britain. It seems to be well proved that most of the species will survive a winter in which the temperature does not fall lower than 23° Fahr. How fortunately circumstanced is the culture of the tree at Rome, may be learned from the fact that the mean lowest temperature registered at the observatory of the Roman College during the years 1863-'74 was 23·48°. Once only in those years a cold of 20° was registered, and even that does not seem to have injured the plants; but when, in 1875, the minimum temperature fell to 16°, the result was the loss in a single night of nearly half the plantation of the year.

But when, as at Tre Fontane, the conditions of growth are on the whole favorable, the rapidity of that growth approaches the marvelous. The mean height, for example, of three trees chosen for measurement by M. Vallée in 1879, was twenty-six feet, and the mean circumference twenty-eight inches. These trees had been planted in 1875, or in other words were little more than four years old. Other trees of eight years' growth were fifty feet high and nearly three feet in circumference at their largest part. These figures refer to Eucalyptus globulus, which certainly grows faster than the other species; and it must be remembered that in warmer climates the growth is even still more rapid. I have seen, for example, trees of Eucalyptus resinifera at Blidah in Algeria which at only five years old were already quite sixty feet high.

The question of how and why the eucalypts exercise sanitary changes so important as those which have been effected at this little oasis in the Campagna, may be best answered when two remarkable properties which characterize many of the species have been shortly considered. The first of these is the enormous quantity of water which the plant can absorb from the soil. It has been demonstrated that a square metre—which may roughly be taken as equal to a square yard—of the leaves of Eucalyptus globulus will exhale into the atmosphere, during twelve hours, four pints of water. Now, as this square metre of leaves—of course, the calculation includes both surfaces—weighs two and three quarter pounds, it will be easily seen that any given weight of eucalyptus-leaves can transfer from the soil to the atmosphere nearly twice that weight of water. M. Vallée does not hesitate to say that under the full breeze and sunshine—which could necessarily form no factor in such accurate experiments as those conducted by him—the evaporation of water would be equal to four or five times the weight of the leaves. One ceases to wonder at these figures, on learning that it has been found possible to count, on a square millimetre of the under surface of a single leaf of Eucalyptus globulus, no less than three hundred and fifty stomata or breathing-pores. And it now begins to be intelligible that, if such an enormous quantity of