10 



the presence in the juices of the plant of some substance which 

 attracts the parasite. On this view infection is independent of the 

 health of the plant, but it has been found that " susceptibility can 

 be induced by various kinds of mechanical injury'' and also by 

 certain interferences with the normal functions such as anaesthetis- 

 ing or overheating the leaf. Often stem-boring insects open the 

 way for hurtful fungi and bacteria. There is a class of fungi — the 

 wound parasites — which can only obtain a hold on the plant when 

 the bark has been injured. Animals frequently denude considerable 

 areas on certain kinds of trees : tar should be liberally applied in 

 such cases. 



The interference of man with cultivated plants is to submit them 

 to conditions different in many ways from those surrounding the 

 plants in their wild states, and to develop them along particular lines, 

 thus disturbing the normal equilibrium and rendering the plant more 

 susceptible to disease. This may have a lot to do with the violent 

 epidemic character frequently assumed by some diseases of cultivated 

 plants. 



The economic pursuit of the greater number of cultivated plants 

 is for their ripened fruits, or for some part connected with repro- 

 duction, such as tubers and fleshy roots. This would not appear at 

 first sight to be very harmful as the production of seed is the great 

 feature in the life of a plant, one might truly say the aim and object 

 of its existence. But man is not content with the quality and 

 quantity of seed which a plant produces in its wild state. He seeks 

 to make various modifications which leave the plant more prone to 

 disease. The balance of a tree's functions is more disturbed where 

 something other than the fruit is the object, such as latex in rubber- 

 producing trees, and various unhealthy signs may make their 

 appearance. In all trees there is a considerable power of respond- 

 ing to changed surrounding and repairing injuries, but some kinds 

 possess it to a greater extent than others. Tapping rubber trees 

 must be looked on as an injury to the tree, but the Para rubber tree 

 (Hevea brasiliensis) appears to be far from delicate and to manifest 

 a great plasticity in adapting itself to new conditions; but this pro- 

 perty cannot be too far presumed upon. Enough information has 

 not yet been collected to specify the effect of tapping on the health 

 of the tree, but on theoretical grounds the action of many Planters 

 in giving the tree a couple of months' rest, especially during the 

 leafless period, is commendable and is probably necessary to restore 

 the functions to their normal state. It is well known that tapping 

 Rambong (Ficus elastica Roxb.) and Ceara (Manihot Glaziovii 

 Mull-Arg.) after the manner practised on Para has had fatal results, 

 even when the cambium remained uninjured. 



In a Para tree on the inside of the cortex, the part containing 

 the latex-vessels, there is a thin layer called the bast, which is a 

 kind of highway along which food products elaborated in the leaf 

 travel to the root and other parts of the tree. Inside this is another 

 thin layer, the cambium, which has the power of growth ; it adds to 

 the wood on the inside and to the bast on the outside, thus 



