302 SCIENCE PROGRESS. 



sooner or later make their appearance. Forms which have 

 been regarded as harmless suddenly assume dangerous 

 parasitism, or those which have hitherto attacked other plants 

 extend their destructive sphere — e.£., BoUytis cinerea and 

 the vine. The conditions of cultivated plants render them 

 peculiarly liable to the attacks of enemies. They are 

 grown in large numbers, close together, so that the spread 

 of disease is unchecked. Certain characters are developed 

 in such plants by human selection : such are the great 

 increase in size of parts (turnip root), the increased paren- 

 chymatosis of organs (cauliflower), and the accumulation of 

 substances in the cell sap (beetroot, sugar-cane). All of 

 these tend to disturb the equilibrium of a healthy condi- 

 tion. On the other hand, the selection of disease-resisting 

 varieties is not prosecuted until a specific disease arises. It 

 is not surprising then that, when diseases appear, they 

 assume the greatest virulence, and frequently large areas 

 are destroyed in a very small space of time. 



The sugar-cane has peculiarities of growth and sub- 

 stance which render it specially liable to such attacks. 

 Instead of the dry stem and soft leaves of most herbaceous 

 plants, it has a juicy stem and tough, dry leaves. Gnawing 

 and burrowing animals eagerly penetrate the hard rind ; 

 and free passage is thus offered for the spores of fungi. 

 The mode of propagation of the sugar-cane is vegetative, 

 by the rough and ready method of cutting pieces off the 

 parent shoot ; and by this means disease may be readily 

 transmitted. There is no cessation of growth, as in colder 

 climates, where many diseases are thus annually reduced. 

 The plants are grown so close together that no other herb 

 can exist in the fields when the canes have reached a 

 certain age. The sugar-cane has been cultivated for 

 thousands of years : it is not likely that the prevalence 

 of disease is a new phenomenon in the fields ; and, as we 

 shall presently see, this is by no means the case. While 

 singularly open to the attacks of parasites, the sugar-cane 

 has great reproductive power. The plant body is divided 

 into joints, each of which is provided with a resting bud, 

 and each bud or eye may develop into a new plant upon 



