EXERCISE 2 



HOW PLANTS GROW 



Statement. Plants are made of cells which vary in age, size, shape, and content. The mature 

 cells may divide into two or more new cells. These in turn absorb food from the sap of the plant, 



develop, mature, and may divide into new cells. This is growth. 



Growth can take place only in the young tissues of the plant. If the cell 

 walls become hardened, as in the heartwood of a tree, the cells no longer divide 

 and that part of the plant ceases to grow. Growth in length of roots takes 

 place at the tip, and the elongating region at most extends over only a small 

 fraction of an inch. Growth in length of stems, however, is much less con- 

 fined. It begins at the tip but the elongating region may extend eighteen 

 inches or more back of the tip. Increase in diameter in some plants is accom- 

 plished by a layer of growing cells which lies between the bark and wood, as in 

 the case of trees. In other plants new tissues originate at the tip and the 

 increase in diameter as a rule takes place only by the enlargement of the cells 

 of these tissues. The growth of these cells soon ceases and the diameter of 

 the stem is limited, as shown by the corn stalk. 



Object. To observe where the growth in roots and stems of various kinds 

 of plants occurs. 



Directions, i. Place kernels of corn between the folds of blotting paper. 

 Keep the blotting paper moist and covered with a cloth until the grain has 

 germinated and the roots are one-half to one inch in 

 length. Beginning at the tip of each root of the 

 sprouted kernels mark fine ink lines across them one- 

 eighth inch apart, being careful not to injure the roots. 

 Replace the kernels in the germinator and allow the 

 plants to grow for a day or two. Explain the results. 



2. In like manner mark lines beginning at the stem tips on other plants 

 after they are sufficiently developed. The bean is a good plant to use. 



3. Obtain a young bean plant after it has put forth leaves and mark 

 lines one-eighth inch apart along the edge of the leaf and on the leaf stem 

 or petiole. In a few days observe the results. Compare a leaf with a plant 

 stem in the way it increases in length. Perform the same exercise with a 

 corn leaf. 



Questions. What part or parts of a young plant are first to appear from 

 the seed ? Explain in what ways knowledge of the manner of growth of the 

 roots is important and why it is difficult to maintain the stem growth inde- 

 pendent of a good root growth. Has this fact any bearing upon the relation 

 of soil texture to root development? Fig. 3. Bean plant 



References. Waters, H. J. Essentials of Agriculture, p. 193. Ginn and Company. Hunt and Burkett. 

 Soils and Crops, pp. 187-188. Orange Judd Company. Bergen and Caldwell. Introduction to Botany, 

 pp. 20, 67. Ginn and Company. 



Fig. 2. Corn plant 



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