THE EUCALYPTUS IN THE ROMAN CAMP AG N A. 99 



sometimes rage in the Campagna. The other great enemy of the tree 

 is cold, and this offers an almost insurmountable obstacle to its success- 

 ful 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 low- 

 est 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 marvel- 

 ous. The mean height, for example, of three trees chosen for measure- 

 ment by M. Vallee in 1879, was twenty-six feet, and the mean circum- 

 ference 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 cir- 

 cumference 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 atmos- 

 phere, 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 at- 

 mosphere nearly twice that weight of water. M. Vallee does not hesi- 

 tate to say that under the full breeze and sunshine which could 

 necessarily form no factor in such accurate experiments as those con- 

 ducted 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 milli- 

 metre 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 



