6 FIELD CROP PRODUCTION 



siderable variation in size or other characters ; thus 

 oats, wheat, barley, and even corn, each of which belongs 

 to a different genus, are all members of the grass family. 

 But the variation between oats and the meadow grasses 

 like timothy is not so great as would appear without 

 examination of the plants themselves. Examination will 

 reveal the similarity in the character of growth, shape, and 

 structure of the leaves, stem, and flowers. The nearness 

 of their relationship will become more evident if one of 

 them is compared with a species of another family, such 

 as one of the clovers. Immediately a great dissimilarity 

 between these will be noticed in the leaves, stem, roots, 

 and flowers. 



7. Orders, classes, and divisions. — As genera and 

 families have been formed, so are the closely related 

 families grouped into orders. Orders with similar charac- 

 ters are grouped into classes, and similar classes form 

 divisions. The division represents the largest group of 

 the plant kingdom. 



8. Divisions. — The plant kingdom has been divided into 

 four great divisions, namely, Thallophytes, Bryophj^tes, 

 Pteridophytes, and Spermatophytes. The Thallophytes 

 and Bryophytes comprise the lower forms of plants ; the 

 algae, fungi, and bacteria belonging in the former group, 

 and the liverworts and mosses in the latter. The plants of 

 these two divisions have neither true stems nor leaves, nor 

 do they produce flowers and seeds. The Pteridophytes 

 include the ferns and related plants. The plants of this 

 division have stems and leaves, but do not produce true 

 flowers or seeds. These three great divisions of the plant 

 kingdom, Thallophytes, Bryophytes, and Pteridophytes, 

 are often grouped together into the " flowerless plants." 

 The one remaining group, namely, the Spermatophytes, 



