220 THE MOVEMENT OF WATER THROUGH THE PLANT 



stems and the relation of these processes to the production of the primary and 

 secondary tissues of the stem is considered more in detail in Chap. XXXI. 



Secondary xylem and secondary phloem also develop from the cambium 

 in most herbaceous dicot stems but in such species the cambium in any one 



stem is never active for more than 

 . RADIAL AXIS — , Qfjg growing season. 



The spring formed xylem 

 tissue, as viewed in cross section^ 

 is usually distinctly different in 

 aspect from that formed later in 

 the season. In many angiosperms 

 the "spring" wood contains more 

 and larger vessels and the cell 

 walls are generally thinner than 

 in the subsequently produced 

 "summer" wood. In the conifers 

 the spring formed tracheids are 

 thinner-walled and of larger 

 cross-sectional diameter than 

 those formed later in the grow- 

 ing season. The transition from 

 spring to summer wood is often 

 a very gradual one. On the other 

 hand the more open xylem tissues 

 formed each spring abut directly 

 upon the denser tissues which 

 were produced during the pre- 

 ceding summer, thus giving rise 

 to an abrupt line of demarcation 

 between the zones of xylem 

 formed in any two successive 

 seasons. The result of this growth 

 behavior is that a cross section 

 of the trunk or a branch of any 

 tree appears as a system of con- 

 centric layers, the so-called annual rings, each representing an annual incre- 

 ment of growth. In rare cases no annual rings nor more than one annual 

 ring may be produced in a season, but usually each ring represents the xylem 

 produced by the activity of the cambium during one season. 



In many woody species as the xylem tissues increase in age important 



CD E 



Fig. 55. Diagram illustrating formation 

 of phloem and xylem elements from the cam- 

 bium. {A) a cambium initial, (B) division 

 of cambium cell, (C) outer cell resulting 

 from division becomes a phloem element, {D) 

 another division of the cambium cell, {E) 

 outer cell resulting from second division be- 

 comes a xylem element. Actually several 

 xylem or several phloem elements are often 

 formed successively. Upon maturation most 

 of the xylem and phloem elements acquire 

 sizes and shapes ^vhich are very different 

 from those of the cambium initials from 

 which they originate. 



