THE STRUCTURE OF PLANTS. 15 



marked. Sometimes it is expanded into a wing", and at others, as in the 

 Milkweed, furnished with a tuft of hairs which serve to waft it from 

 place to place. The mark left by the separation of the seed-stalk or 

 funiculus is called the Hilum, which is conspicuous in the Bean and 

 Horse-chestnut. 



53. SYSTEMATIC BOTANY is that branch of the science which groups 

 together plants according to their resemblances in structure. Individual 

 plants that are so much like each other that they may be conceived to 

 have a common origin, are comprised under the head of Species. Plants 

 are apt to vary much, from local influences, and to deviate somewhat 

 from the regular form of the species ; these departures from the typical 

 form are considered as Varieties. Where varieties perpetuate their pecu- 

 liarities from one generation to another by the seed, they form Races ; of 

 which our cultivated plants furnish numerous examples. Those species 

 which have many points of resemblance, though differing in minor char- 

 acters, are grouped together in Genera, and genera again are collected 

 into Families or Orders, these into Classes founded upon fundamental dif- 

 ferences in the structure of the embryo, stem, &c. The next group in 

 the ascending order is that of Series, where the whole vegetable kingdom 

 is separated into two great series, the one comprising the Flowering and 

 the other the Flowerless Plants. 



54. In the descriptions of plants, the account given of the Order should 

 apply to all the genera included in it ; that of the Genus should include 

 the important characters of all the species it comprises ; and that of a 

 Species should present those points which distinguish it from other 

 species in the same Genus. 



The names given to plants are double, corresponding to the surname 

 and baptismal names of persons. The name of the Genus is placed first, 

 followed by that of the species ; the latter is usually in the adjective 

 form. 



55. Of course all the plants that one meets with will not be found in 

 this work, it being intended only to include those which are to be found 

 in cultivated sections. In order to find the name and description of any 

 particular plant, the first thing to be settled, (it being of course a flow- 

 ering plant.) is, to which class to refer it ; this is usually indicated by 

 the leaves, or, at any rate, it may be ascertained by making a cut across 

 the stem. If it be an exogenous stem, then it must be ascertained 

 whether the corolla is present, and if present, whether it is composed of 

 many pieces or is a more or less entire single one. These preliminaries 

 being settled, a reference to the Key and a little patience will soon de- 

 termine the Family to which it belongs. If upon referring to the 

 description, the plant in question agrees with the character given to the 

 Family, then the genus is to be ascertained, and after this the species. 



