OF ROOTS FROM LAYERS. 197 



same season, a portion of the bark, immediately at the base of the tongue 

 of the layers, without taking them out of the ground. 



By the preceding mode of management, the ascending fluid is permitted 

 to pass freely into the layer to promote its growth, and to return till the 

 period arrives at which layers generally begin to emit roots ; the return 

 of the sap through the bark is then interrupted, and roots are, in conse- 

 quence, emitted ; and I entertain little doubt that good plants of trees, 

 of almost every species, may be thus obtained at the end of a single season. 

 I wish it, however, to be understood, that my experiments have been 

 confined to comparatively few species of trees ; and that I am not much 

 in the habit of cultivating trees of difficult propagation. 



XXIV. ON THE PREVENTION OF THE DISEASE CALLED THE CURL IN 



THE POTATOE. 



[Read before the HORTICULTURAL SOCIETY, February '2nd, lf!13.] 



THE rough and uneven surface of the leaf, which in excess, indicates, 

 and indeed constitutes, the disease called the curl in the potatoe, appears 

 to exist in, and to form an essential characteristic of, every good variety of 

 that plant ; for I have never found a single variety, with perfectly smooth 

 and polished leaves, which possessed any degree of excellence ; and I 

 have endeavoured to prove, in a former communication *, that the 

 rough and crumpled state of the leaf probably originates in the pre- 

 ternaturally inspissated state of the fluid, in the firm and farinaceous 

 potatoe. Those varieties are, however, generally most productive and 

 grow with the greatest luxuriance, of which the leaves are smooth and 

 polished ; and this point tends to prove, that the smooth leaf is a more 

 perfect and efficient organ than the rough one ; the latter indicating 

 some degree of approximation to disease. 



I have stated, in another paper f , that I obtained a second crop of 

 potatoes by planting those of an early variety in the same soil from 

 which a crop of the same variety had been taken, in the month of 

 July ; and that I had employed, with success, the tops of those taken 

 up, with green fern and nettles, as manure. But I found the tubers 

 produced by those last planted to be much more soft and watery, 

 when boiled, than others of the same variety, and consequently much 

 inferior in value for every culinary purpose ; and therefore, these were 



* See page 185. f See page 194. 



