Buds and Stems. 



57 



stems of plants in general ; but in the roots, where the 

 stress is applied more nearly as a straight pull, we find the 

 strengthening elements in the form of a compact column 

 at the center, a form which is best adapted to resist a 

 longitudinal pulling stress. In the distribution of their 

 strengthening elements, plants have been obliged to take 

 account of many things, such as economy of materials, 

 the free circulation of materials, and the formation of 

 branches. 



40. Conduction of Materials. — The conducting elements 

 of the stem are quite as important as the strengthening 

 elements. The leaves give off large amounts of water by 

 transpiration, and must, therefore, constantly receive com- 

 pensating supplies from the roots. To facilitate the pas- 

 sage of water, long tubes are provided, extending from the 

 roots into the leaves. These tubes, known as tracheal 

 tubes, have relatively large openings, and their walls are 

 thickened in various ways (see Fig. 21) so as to strengthen 

 them, and at the same time leave thin places for the pas- 

 sage of water into the tissues along their route, and to 

 allow the passage of food materials into the tubes while 

 they are yet in state of formation. The thin places in the 

 tubes also serve another purpose every spring, in permit- 

 ting the reserve food materials in the underground parts 

 and lower regions of the stems to pass into the tubes and 

 be carried rapidly upward to the unfolding buds by the 

 ascending currents of water. The tracheal tubes have 

 cross partition walls in them which are about eighteen 

 inches to three feet apart; these are thin, however, and 

 do not much retard the passage of the water. 



The tracheal tubes lie in the xylem portion of the vas- 

 cular bundles and are thus within the cambium ring. 

 Accordingly when the bark is removed from a stem, the 



