194 BOTANY. [CHAP. VH. 



covered thousands of acres with a rank growth, greatly 

 to the disadvantage of the sheep- farming industry. A 

 similar example on a smaller scale followed the accidental 

 introduction of the American water-weed into Britain, 

 which at one time threatened to choke up many of our 

 canals and rivers, but in both the above-mentioned 

 cases, and also in many others, after the first spurt made 

 by an alien on finding itself in what to it proves to be 

 virgin soil, a gradual decrease in its vigour of growth 

 and consequent distribution takes place, and it settles 

 down to the level of having to hold its own in the 

 struggle for existence, or in some instances almost 

 entirely disappears. Similar waves of inundation by 

 parasitic plants, more especially microscopic species of 

 fungi, have been observed, which after causing serious 

 devastation for a certain period, recede, either altogether 

 or to such an extent as not to materially influence the 

 continuance of the species of plant on which they are 

 parasitic. In connection with the above it is sufficient 

 to note the potato disease, due to the ravages of a 

 minute fungus known as Phytophthora infestans ; the 

 hollyhock disease, caused by an equally minute fungus 

 called Puccinia malvacearum; finally, the destructive 

 silk-worm disease known as (< muscardine " is also due 

 to the ravages of a minute fungus, Botrytis Bassiana, 

 which attacks and destroys the living insect. 



As already stated, every plant of high organization is 

 much influenced by temperature ; in other words, a 

 certain amount of temperature is necessary to enable it 

 to perform all its functions properly. Hence we speak 

 of a palm region, cactus region, etc., meaning that the 

 particular group mentioned is highly characteristic and 



