44 PEACTICAL BOTANY 



43. Growth in length. Under favorable conditions the 

 younger portions of the stem for a good while increase con- 

 tinually in length. The rate of growth varies greatly in dif- 

 ferent plants : sunflowers and giant ragweed (Artemisia) may 

 grow to a height of 10 or 12 feet, and climbers like gourds 

 and hops to a length of perhaps 40 feet, in a single summer. 

 On the other hand, pine seedlings during their first summer 

 only grow to be from 1 to 3 inches high, and oak seed- 

 lings less than 5 inches. The growth per year for a time 

 continues to increase and then diminishes. For example, the 

 long-leaf pine (Fig. 261) grows only about three quarters 

 of an inch the first year. For the first fifty years it makes 

 an average annual growth of 14 or 15 inches; for the next 

 fifty, 4 or 5 inches ; and from one hundred years to extreme 

 old age, about one and one-half inches. It usually lives about 

 two hundred years. 



The growth of the younger nodes of most plants is quite un- 

 equal, as may be learned from the study of a rapidly growing 

 stem, such as the morning-glory. 1 It will also prove interest- 

 ing to measure such plants as corn, broom corn, hemp, and 

 pole beans, to determine whether they elongate more during 

 the day or the night, and during warm or cool weather. 



44. Internal structure of the young dicotyledonous stem. 2 

 The structure of the fully developed stem can best be under- 

 stood by tracing its development from the time when the em- 

 bryo begins to grow in the sprouting seed. That, however, is 

 a rather difficult process to follow, so this brief account will 

 begin with the stem already considerably developed. 



In common language the dicotyledonous stem is said to con- 

 sist of bark, wood, and pith. These regions are very distinctly 



1 See Bergen and Davis' s Principles of Botany, p. 17. 



2 See also Sects. 45-48. The stem of many gymnosperms (e.g. trees of 

 the Pine family) in its general structure much resembles the dicotyledon- 

 ous stem. For a general account of the stem structure of dicotyledons and 

 monocotyledons see Coulter, Barnes, and Cowles's Textbook of Botany, 

 chap, iv, A. ANGIOSPERMS. 



