210 NATURAL HISTORY. 



plants, however, this difference is fairly manifested. 

 First appears a slender, succulent stam, which soon 

 divides into two principal parts, outer and inner ; the 

 outer rind or cortical portion is called bark, the inner 

 or ligneous, is termed wood ; both, although at first soft, 

 gradually dry and become hard, after which they serve 

 for the support and protection of the plant, rather than 

 for its nourishment. The portion in which are placed 

 the organs of nutrition lies between the bark and wood ; 

 it remains unchanged and soft during the life of the 

 plant, and is called alburnum or sapwood. This drying 

 up every year adds alike to the thickness of the bark 

 and wood, and thus, as layer after layer is formed annu- 

 ally, the plant increases in circumference, and on being 

 sawed across will be found to consist of a number of 

 rings, by which its age is determined. Not so, however, 

 with the bark, for, although a new layer is annually 

 added, instead of forming ligneous layers like the albur- 

 num, after having once served the purpose of circulation, 

 it is thrown aside, or rather divides, as the girth of the 

 plant increases, into rifts or fissures, as if forced to do 

 so. by the accumulating growth within. 



The most important part of a plant, however, is its 

 flowers, for not only do they bestow pleasure from their 

 brilliant colorings, beautiful structure, and sweet odor, 

 but are also most useful as being the precursors of the 

 fruit. And besides these, it is the flowers that bear the 

 seed which is to perpetuate the race, and are the charac- 

 teristic tokens of the species to which they belong. 



Whilst animals are always found divided into the two 

 races of male and female, in the vegetable world the sexes 

 are frequently found united in one flower, and the organs 

 peculiar to each are plainly observable. In a tulip, for 



