The embryo develops from a single fertilized "egg," growing by cell division into two, four, eight, and a constantly increasing number of cells until after a time a baby plant is formed, which as in the bean, either contains some stored food to give it a start in life, or, as in the corn, is surrounded with food which it can digest and absorb into its own tiny body. We have seen that these young plants in the seed are able to develop when conditions are favorable. Furthermore, the young of each kind of plant will eventually develop into the kind of plant its parent was and into no other kind. Thus the plant world is divided into many tribes or groups. Plants are placed in Groups.--If we plant a number of peas so that they will all germinate under the same conditions of soil, temperature, and sunlight, the seedlings that develop will each differ one from another in a slight degree.[27] But in a general way they will have many characters in common, as the shape of the leaves, the possession of tendrils, form of the flower and fruit. A _species_ of plants or animals is a group of individuals so much alike in their characters that they might have had the same parents. Individuals of such species differ slightly; for no two individuals are exactly alike. Footnote 27: NOTE TO TEACHERS.--A trip to the Botanical Garden or to a Museum should be taken at this time. [Illustration: A colony of trilliums, a flowering plant. (Photograph by W. C. Barbour.)] [Illustration: Rock fern, _polypody_. Notice the underground stem giving off roots from its lower surface, and leaves (_C_), (_S_), from its upper surface.] Species are grouped together in a larger group called a genus. For example, many kinds of peas--the wild beach peas, the sweet peas, and many others--are all grouped in one genus (called _Lathyrus_, or vetchling) because they have certain structural characteristics in common. Plant and animal genera are brought together in still larger groups, the classification based on general likenesses in structure. Such groups are called, as they become successively larger, _Family_, _Order_, and _Class_. Thus both the plant and animal kingdoms are grouped into divisions, the smallest of which contains individuals very much alike; and the largest of which contains very many groups of individuals, the groups having some characters in common. This is called a system of classification. Classification of the Plant Kingdom.--The entire plant kingdom has been divided into four sub-kingdoms by botanists:-- 1. _Spermatophytes._ { _Angiosperms_, true flowering plants. { _Gymnosperms_, the pines and their allies. 2. _Pteridophytes._ The fern plants and their allies. 3. _Bryophytes._ The moss plants and their allies 4. _Thallophytes._ The Thallophytes form two groups: the Algae and the Fungi; the algae being green, while the fungi have no chlorophyll.