DISEASES AND THEIR REMEDIES. 129 



Trees do not depend solely upon their leaves for 

 the supply of carbonic acid. The roots gather a 

 very considerable part of this gas, so essential to 

 plant life, not in a pure state, as is done by the 

 leaf, but in chemical combination with other ele- 

 ments. This is the case especially with trees which 

 have very yellow roots. Such trees send their 

 roots either into a very porous soil easily penetrated 

 by the air, or else send them near the surface, where 

 they find a greater abundance of air, which decom- 

 poses manure and is essential to the formation of 

 carbonic acid. Such is the case with the orange 

 tree and roots. If the tree is planted too deep or 

 the crust on the top of the soil has become very 

 compact, these roots, dependent upon air for health 

 and ability to perform their functions, are virtually 

 smothered. They make an effort to grow, but as 

 often as they form rootlets and root-hairs, these die 

 and convey no nutriment for the formation of the 

 woody structure of young shoots, so the new and 

 tender cells, which are but the frame-work of the 

 plant, perish for want of support. And hence the 

 light cellular structure, in the form of young shoots, 

 dies back as certainly as if it had been cut from 

 the older wood. I have occasionally dug up trees 

 so afflicted and found them wanting in new roots. 

 The remedy is to reset, or else take away the top 

 soil till the lateral roots are brought near the sur- 

 face, and to keep the soil well cultivated. Cut away 

 all diseased wood and roots. When the extremities 



