4 INTRODUCTION. 



soil or situation, and such plants are found in immense num- 

 bers over nearly the whole surface of the earth. The natural 

 families of corn plants or grasses and composites serve as 

 familiar examples. It is well known to cultivators that many 

 plants dwindle, and never become properly developed, if too 

 close together. Corn plants or cereals, indeed, acquire a dis- 

 tinct habit from being grown closely together generation after 

 generation ; and Dame Nature herself seems conscious of this 

 fact, if we think of the many and varied contrivances by which 

 the seeds of plants are dispersed or scattered, so as naturally to 

 secure a change of soil. The seeds of most composite plants, 

 or members of the Thistle and Dandelion family, are furnished 

 with downy appendages (pappus), which are sufficiently buoyant 

 to bear them up in the slightest breeze ; hence they become 

 "as rolling things before the wind." The Squirting 'Cucumber 

 affords another curious illustration of how nature manages to 

 sow her seeds on new ground, so as to secure a rotation of 

 crops, and in this case the slimy seeds are thrown out of the 

 fruit (by the sudden contraction of its tissues) to a distance of 

 several yards ; while our native Gorse or Whin, and the common 

 Broom, afford instances somewhat similar. Where trees and 

 other seed-bearing plants overhang rivers or streams, the seeds 

 often fall in the water, and are carried for many miles by the 

 current, until by some accident they are washed ashore, and 

 germinate to produce seed themselves, which in its turn is 

 carried still further by the tide. The seeds of tropical Orchids 

 are as fine as sawdust, and very much lighter, the least breath 

 of air being sufficient to waft them along, until they lodge on a 

 dead or partially decayed tree or branch, which they not unfre- 

 quently wreathe with their foliage and flowers ; while others 

 vegetate on moss, grass, and other low vegetation. Birds often 

 aid in distributing the seeds of fruit-bearing trees and shrubs. 

 This is particularly the case with the Mistletoe; and the appar- 

 ently wild Gooseberry and Currant bushes found in hedgerows, 

 or on decayed trees and ruins, are not unfrequently propagated 

 by birds, who have carried the fruit from neighbouring gardens. 

 Seeds are not unfrequently rubbed off low-growing vegeta- 

 tion, and carried long distances in the hairy covering of wild 

 animals; and some plants follow the footsteps of civilisation 

 everywhere. What seeds are to the flowering-plants, spores 

 are to Ferns, Mushrooms, and other cryptogamic vegeta- 

 tion. It is true that the spore is not quite the same in struc- 

 ture as a seed, and the manner of its germination is very 

 different, but for all practical purposes they may be considered 

 the same. Many spores are so small scarcely visible, indeed, 



