174 The Plant World. 



a way that they broke, and I could not tell how much longer 

 they might have been. 



I believe that moisture and a mild climate are the essential 

 factors in producing the luxuriant growth of this lichen, but un- 

 doubtedly lack of disturbance by the wind prevents the long 

 threads from being broken and so gives longer individuals. In 

 the coast mountains, and especially on the west slope, this species 

 is more abundant and extends over wider areas, I am told, but 

 I have not observed it there. Here again moisture and mild 

 temperature are factors, the moisture being abundant throughout 

 the year. Of course there is a long growing period, but that is 

 due entirely to the mild climate and moisture combined. With- 

 out either, growth must soon cease. 



One fact that has interested me is that Usneas disappear 

 in towns and along railroad tracks. It is said that smoke from 

 chimneys kills them. The only point I can suggest to substan- 

 tiate this is that along a certain railroad on which wood and 

 coal were burned the Usneas became very rare. A few years 

 ago petroleum was adopted as a fuel and last summer it seemed 

 to me the Usneas were becoming more abundant. Why petrol- 

 eum smoke should be less injurious I can not say, and my con- 

 clusions may be apparent rather than real. 



FORMATION OF ADVENTITIOUS ROOTS BY THE HACK- 

 BERRY TREE. 



By F. A. Wolf. 



An interesting case of the formation of adventitious roots 

 by the hackberry,Ce/^t,$- mississippiensis Bosc, was noticed upon 

 the campus of the University of Texas. This phenomenon is 

 not new, as recently * a very similar example was reported on 

 the Umbrella China tree, but as it is not common, it is worthy of 

 consideration. 



During a storm the trunk of this large hackberry tree had 

 been split and the fallen portion was subsequently removed. 

 At a point about eight feet above the ground and a little above 

 the broken edge of the tree a cluster of fibrous roots were formed. 



•BaU. O. M., Bot. Gax., 46:303-4. 1908. 



