276 PRINCIPLES OF GARDENING. [CH. VIII, 



their study. To define and describe tlieir specific 

 cliaracters is very useful ; but it is chiefly so, be- 

 cause it is like a good index to an intricate volume. 

 It is of far more utility to ascertain their habits, and 

 their periods of gestation and transformation, be- 

 cause such knowledge is that which often affords us 

 one of the best means of avoiding their ravages. 



In cabbages, the ambury may usually be avoided 

 by frequent transplan tings, for this enables the work- 

 man to remove the excrescences upon their first ap- 

 pearance, and renders the plants altogether more ro- 

 bust and ligneous ; the plant, in its tender, sappy 

 stage of growth, being most open to the insect's 

 attacks. 



The sap of the turnip and cabbage thus diseased 

 undergoes a considerable change. Its specific gravity 

 is much increased, arising from an excess of mu- 

 cilage, vegetable extract, and saline constituents, 

 which it naturally contains, caused probably by its 

 being in a concentrated state ; for it is very con- 

 siderably reduced in quantity, compared with what 

 the same plant contains when healthy. The increase 

 of the saline components unquestionably exasperates 

 the disease. They consist chiefly of chloride and 

 carbonate of potass, which, by the corroding power of 

 the last-named, and the irritating qualities of both, 

 must increase the sanious discharge, by stimulating 

 the already lacerated and morbidly sensitive vessels. 



