74 THE STUUGflLE FOB EXISTENCE. 



extension. Then, owing to the rapid expansion of the crown and 

 the more vigorous circumferential growth of the stem and branches, 

 the quantity of food available for the sustenance of undeveloped 

 buds diminishes, with the result that they gradually lose their vita- 

 lity, and perish and ultimately disappear under the constantly ad- 

 vancing layers of woody growth. This diminution of the number 

 of the dormant buds begins at the base and top of the tree and 

 progresses steadily respectively upwards and downwards towards 

 the middle, where however some persist up to theideath of the tree. 



(iii) Repeated temporary arrests of vigour of crown. — Each 

 time there is such arrest, there is a superfluity of assimilated food, 

 which then becomes at once available for the dormant buds. These 

 buds thus receive a strong stimulus, which not only keeps alive those 

 about to perish from want of nourishment, but also strengthens the 

 rest and enables them to multiply very considerably. Some of 

 them may even sprout and thereby give a still further impulse to 

 their immediate neighbours. The arrest of vigour here referred to 

 must be sudden in order that the quantity of food thus suddenly 

 rendered available may be large enough to enable such comparative- 

 ly weak organs as the dormant buds to utilise it. SxKsh arrests, if 

 frequent and sudden enough within certain limits, may give such a 

 preponderance to the dormant buds, that the entire surface of the 

 woody cylinder of the tree or shrub in question may become a 

 mass of warty excrescences formed by those buds, a result that is 

 conspicuously exemplified by mutilated specimens of Celtis Roxhur~ 

 ghii. In the same way a stem hardly a foot in diameter may throw 

 up, when cut back, a clump of more than 100 shoots. 



The arrested vigour of the crown here discussed may come about 

 from one or more of the following causes: — ^- 



1. Drying up of the soil in the hot weather; 



2. Great heat or cold, including hot and chilly winds, and 



excessive insolation and frost; 



3. Conflagrations; 



4. Pollarding and every description of lopping and hacking; 



5. Breaking of a branch from any cause whatsoever; 



6. Depredations of insects and other animals; and 



7. Excessive light. 



All these causes are more fruitful, numerous and effective in this 

 country than in Europe. Thus to abundant rain and a bright hot 

 sun during the season of vegetation succeed cold and even frosty 

 nights in the cold weather; nay, in the cold weather itself, a high 

 forcing temperature during the day is at once followed by a rapid 



