Growth and Movement. 



115 



to meet the most pressing demand at the time ; namely, for 

 a greater water supply for the increased transpiring sur- 

 face of the leaves. Accordingly we find that a great many 

 water tubes are first formed (see Fig. 49), which com- 

 municate with the veins of the leaves and with the water 

 tubes running out into the 

 newly formed rootlets. 



The water supply having 

 been provided for, the cam- 

 bium cells next produce wood 

 fibers in much greater pro- 

 portion. At the close of the 

 season we find on examining 

 the year's growth that it con- 

 sists of a zone of tissues (s) 

 in which the water tubes pre- 

 dominate, followed by a zone 

 (/) which is more dense and 

 firm because the water tubes 

 in it are smaller and fewer, 

 and the thick-walled wood 

 fibers more in evidence. A 

 single ring of annual growth 

 consists then of a zone of 

 early growth and a zone of 



later and denser growth, each zone having its own peculiar 

 significance in the economy of the plant. The cambium 

 ring makes anniial additions to the bark as well as to the 

 wood, but in much less quantity, and the additions are not 

 demarked into rings of growth. 



90. Plants without Annual Rings. — In perennial mono- 

 cotyledonous stems, which branch but little or not at all, 

 the crown of leaves does not increase materially from year 



Fig. 49. 



Constitution of a ring of growth, j and 

 u, early growth ; t, late growth. The 

 lowest row of small cells in s belongs 

 to the late growth of the previous 

 year. The early growth here con- 

 sists c!iiefly of the tracheal tubes and 

 wood parenchyma and the late 

 growth chiefly of wood fibers, s and 

 ^ constitute an annual ring. After 

 Haberlandt. 



