CHAPTER II 



THE PARTS OF PLANTS 



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The principal parts, or organs, of the more famihar seed-bearing plants 

 may be distinguished in a bean plant as it develops from the seed that is 

 planted to the seeds that are harvested. This life cycle from seed to seed 

 — one entire generation of the bean plant — may be completed in a few 

 weeks. From a study of the diagrams in Figs. 8 and 9 the complete life 

 cycle of a bean plant may be visualized. 



Within the seed coat is an embryo consisting of a short stalk (the 

 hypocotyl) and a pair of very large, thick, leaf -like structures (the 

 cotyledons) which are at the first node. At the apex of the hypocotyl is 

 a terminal bud (the plumule), and at the other end is a root tip from 

 which the primary root develops. 



Two or three days after the seed is planted, the primary root begins 

 to elongate. It breaks through the seed coat and penetrates into the soil. 

 From the primary root lateral roots grow more or less horizontally. The 

 young plant is now firmly anchored, and the rapid growth of the hy- 

 pocotvl lifts the cotyledons well above the soil surface. 



As the cotyledons increase slightly in size the broken seed coats fall 

 away. From the plumule a stem bearing leaves begins to develop. A 

 lateral bud soon becomes visible in the axil of each leaf. Each bud con- 

 sists of a stem tip bearing embryonic leaves. The first true leaves that 

 develop at the second node of this stem are simple, opposite, and heart- 

 shaped. From the terminal bud another stem segment ( intemode -\- 3rd 

 node ) grows, and from this third node only one leaf forms — this time a 

 leaf of three leaflets. Growth in height continues by the development of 

 additional stem intemodes and new leaves at the nodes until 5 or 6 

 leaves are formed. In the meantime, leaf-bearing branches may also be 

 developing from the lower lateral buds. These three distinct parts — the 

 roots, the stems, and the leaves — constitute the vegetative organs of the 

 plant. Internally they are all interconnected by veins, or vascular bundles 

 (Fig. 10). 



Usually after the formation of leaf-bearing branches, another type of 



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