344 



A TEXT-BOOK OF BOTANY. 



season's growth are distinct from those already formed during 

 the previous season in both color and structure of the wood (espe- 

 cially in the thickness of cell-walls and the width of the tracheae 

 or vessels), we perceive in this manner distinct zones of wood, 

 or the " annual rings " as they are called, the larger vessels with 

 thin walls being produced in the spring and early summer. 



Various abnormal stem-structures are known which are due 



pv j-s^i^i 1 .u, \^aa if^er (Wrii \J' 





FIG. 198. Transverse section of wood of Rhamnus Frangula showing that the annular 

 rings seen in woody portion of plants is due to a difference in the nature and structure of the 

 cells formed in the spring and in the fall. In the spring numerous large tracheae or vessels 

 (v) are formed, whereas in the fall very few vessels and mostly wood fibers are developed, 

 the cells of these being smaller as they approach the end of the year's growth. After Rossmann. 



to certain peculiarities in the growth in thickness of stems. These 

 are especially noticeable in lianes. In some of the Monocotyledons, 

 as in Dracaena, Yucca, Agave and Aloe, we find a secondary in- 

 crease in growth of the stems. 



In summarizing the root and stem structures of Monocotyle- 

 dons and Dicotyledons the following general facts should be 

 borne in mind. Monocotyledonous stems resemble Monocotyle- 

 donous roots except that the fibrovascular bundle or mestome 



