THE VEGETABLE KINGDOM. 485 



A knowledge of the fundamental conditions demanded by 

 vegetation explains certain phenomena which have occa- 

 sionally astonished the vulgar. When these conditions are 

 wanting, seeds are often preserved torpid for a long time in 

 the place which incloses them, and then, when they find 

 themselves under the influence of favorable circumstances, 

 they cover a site with a form of vegetation unknown there 

 within the memory of man. 



Thus, according to the account of Ray, after the great 

 fire of London, the hedge-mustard {Sisymbrium Irio) all at 

 once grew thickly on the ruins of this city, where previously 

 it was unknown. When certain forests are burned we see 

 plants spring from their soil which were never previously 

 known there. Analagous facts have been noticed after old 

 marshes have been dried up. Their beds, laid bare, are 

 sometimes covered with an entirely new form of vegeta- 

 tion, quite unknown in the country, and arising doubtless 

 from seeds having been buried under the water and pre- 

 served there till, having been exposed to the air, all the 

 conditions necessary to germination, which were previously 

 wanting, were now brought to bear upon them. 1 



1 Thompson's weed {Lepidium Draba), a plant which gives much annoyance to 

 agriculturists, appears to have been introduced in the straw of the beds brought 

 back from the disastrous expedition to AValcheren. The troops being disem- 

 barked at Ramsgate, the beds were ripped up and the straw thrown into an old 

 chalk-pit belonging to a Mr. Thompson. It was subsequently used as manure, 

 and wherever this manure was laid down a plentiful crop of the new weed was 

 the result. This weed has now spread over a great part of Kent Popular Sci- 

 ence Revieio, vol. v., p. 492. Tk. 



