STUDIES OF SEED-PLANTS 163 



of roots at the base of the stem,- but each one of these can 

 grow a new stem only at the upper or stem end. Many of 

 the root-like structures from which new stems originate 

 are really underground stems (see 176). A familiar ex- 

 ample is the ordinary potato. 



STEMS OF SEED-PLANTS 



We have seen in the case of the bean plant that the 

 stem has certain definite functions in the life of the plant. 

 Most of the statements made concerning the structure 

 and functions of the stem of the bean plant ( 69) are 

 true of higher plants in general. However, stems of many 

 plants have modified structure and special functions; and 

 we should have a very limited knowledge and appreciation 

 of plant stems if we confined our study to one kind of stem. 

 Hence, the following lessons provide for study of the more 

 important facts concerning stems which have not already 

 been presented in connection with the bean plant. 



156. Twigs. This is a popular term applied to pieces 

 of stems, or to small branches of trees and shrubs. They are 

 very convenient for study, because they illustrate on a 

 small scale the general structure of the entire stem of the 

 plant from which they are taken. 



Horse-chestnut Twig. (L) Or twigs of hickory, ash, box-elder, 

 ailanthus, linden, sycamore, etc. Note that the bud at the end of 

 the twig (terminal bud) is larger than the buds along the sides 

 (lateral buds). Observe the positions of the buds along the stem 

 are they alternate or opposite to each other? What is the relation 

 of the arrangement of each successive pair of buds to each other ? 

 Observe the large horseshoe-shaped scars immediately beneath the 

 buds. These are leaf-scars. What is the meaning of the markings 

 on the scars ? What is the position of a bud with reference to the 

 leaf ? Look for other kinds of scars on the twig. The very small 

 scars scattered irregularly over the stem are lenticels or breathing 

 pores ( 105). The nature of the rings of scars will be made evident 

 after a study of the bud. Often one can find a large scar in the fork 



