PLANTS ARE PLACED IN GROUPS 179 



ground used by plants in food manufacture. Nature's law tells 

 us that food cannot be made without certain raw materials. One 

 of the basic elements needed in protein food manufacture is 

 nitrogen, which is present in the air but is not available for use. 

 One kind of bacteria is able to take it from the air, nature's store- 

 house, and to fix it in a form usable by green plants. These nitrogen- 

 fixing bacteria are among man's best friends. 



PROBLEM I. HOW DO WE CLASSIFY PLANTS? 



Laboratory Exercise. Collect a number of common plants in flower 

 at the time you take this work, and see if, by using the information you 

 find later on in this unit, you can identify them. Use all the help you 

 can get, such as simple keys which are found in any good botany or in 

 popular books on identification of flowers. 



Plants are placed in groups. If we plant a number of pea seeds 

 so that they will all germinate under the same conditions of soil, 

 temperature, and moisture, the seedlings will differ one from 

 another in a slight degree. But in a general way they will have 

 many characters in common, such as the shape of the leaves, the 

 length of tendrils, and the form of the flower and of the fruit. More- 

 over, if the seeds from these peas are planted, they in turn will 

 give rise to plants which will closely resemble the parent plants 

 from which the seeds came. Such plants are said to belong to the 

 same species. A species is a small group of plants or animals hav- 

 ing certain characteristics in common that make them different 

 from all other plants or animals. Similar species are placed 

 together in a larger group called a genus (plu. genera). For 

 example, many species of peas — the wild peas, beach peas, sweet 

 peas, and many others — are all grouped in one genus called 

 Lathyrus (lath'i-rws) because they have certain structural char- 

 acteristics in common. 



Practical Exercise 1. Give a good definition of a species ; of a genus. 



Genera of plants or of animals are brought together in still 

 larger or more inclusive groups, the classification being based on 

 general likenesses in structure. Such plant and animal groups 

 are called, as they become successively larger, family, order, class, 

 and phylum. This is called a system of classification. 



