January 30, 1920] 



SCIENCE 



119 



trouble manifested itself in. the change of 

 color of the foliage, the leaves turning brown 

 as if burned or killed by frost, and drying out 

 and presently beginning to fall. The leaves 

 which fell showed no sign of fungus or bac- 

 terial infection. We were therefore forced to 

 conclude that the trouble was further down 

 and we were compelled by the condition of 

 the trunk to suspect that the difficulty was 

 either between the trunk and the leaves or 

 below ground. As we had no convenient 

 means of climbing the trees to make any ex- 

 amination of the branches, we concluded to 

 look at the roots first. 



By laying bare the more superficial part of 

 the root system with pick and shovel, we 

 found that the large superficial roots had 

 been broken through at various distances 

 from the trunk by the heavy plows which, 

 up to that time, had been used in the spring, 

 for a number of years, to clear the ground 

 imder the trees of weeds. The deep plough- 

 ing had resulted in the serious injury, the 

 wounding or amputation, of all the roots to a 

 distance of twelve or fourteen inches below 

 the surface. In this way the roots, absorbing 

 moisture from the upper layers of the soil, 

 were either very seriously limited, or ab- 

 solutely destroyed, as regards their capacity 

 for absorbing water ; and the soil water supply 

 of these trees came therefore through the 

 taproot or its deeper branches and from the 

 branches running vertically downward from 

 the imderside of the uninjured lateral roots, 

 from distances below the surface, of which 

 we have no means of knowing anything. 

 Whether one half or what other proportion of 

 the absorbing surface of the root was thus 

 destroyed we also have no means of knowing. 

 The condition of the roots led us to suspect 

 that this might be the cause of the condition, 

 deplorable in appearance, of the blue gum 

 trees throughout the Arboretum. 



We were confirmed in this suspicion by ex- 

 amining the root system of the Monterey 

 cypress {Oupressus macrocarpa) tree growing 

 close to the big eucalyptus tree previously ex- 

 amined. We were interested to find that the 

 horizontal roots of the Monterey cypress grew 



enough deeper in the soil entirely to escape 

 the heavy plows which had wounded or am- 

 putated the roots of the eucalyptus. This 

 Monterey cypress tree presented none of the 

 deplorable features of the eucalyptus trees, 

 for although its foliage was dusty, it was 

 green and far from dying. We therefore con- 

 cluded that the trouble with the big blue gum 

 trees of our Arboretum was lack of water, 

 due to an impaired root system. 



That this suspicion was justified we believe 

 is confirmed by two additional observations. 

 Many of the eucalyptus trees which were evi- 

 dently dying, as indicated by the brown color 

 of the leaves, were cut down. Those that 

 were cut down early enough, promptly stump 

 sprouted, and have since grown up into prom- 

 ising young trees, borne on the old butts. By 

 thus drastically reducing the evaporating sur- 

 face, the water absorbed by the roots was con- 

 served and the quantity became immediately 

 adequate to meet the loss. Additional con- 

 firmation of our suspicion has been furnished 

 during the last two years. 



In the winter of 1917-18 there fell in Palo 

 Alto scarcely more than eight inches of rain. 

 In the following autumn there was no sign 

 of injury among the eucalyptus trees, of 

 which there were still many in the Arboretum. 

 To be sure, raany of the larger and finer had 

 been cut five years earlier, but enough were 

 left to show damage if the damage had been 

 present, for the rainfall in the rainy season 

 of 1917-18 was about an inch less than in the 

 fifth year preceding. Furthermore, although 

 the rainfall in Palo Alto in the rainy season 

 of 1918-19 was approximately twenty-three 

 inches, there has been practically no rain 

 since early March until late September; and 

 there is not yet a total of one inch of rain in 

 the immediate vicinity of the Arboretum, 

 though there is no sign of drought among the 

 eucalyptus trees. 



The manner of keeping down the weeds in 

 the Arboretum, however, has been changed, 

 since our observation of the injury due to 

 deep ploughing, and the disk harrow or spring 

 tooth harrow are all that are used for cutting 

 down and keeping down the weeds which are 



