5 8 



GROWTH AND WORK OF PLANTS 



complete ring around in the stem, at this point separating the bast 

 and wood of all the bundles. The bundle is not therefore closed 

 but is open. This is characteristic of the bundles of the dicoty- 

 ledons as distinguished from those of the monocotyledons. The 

 stems of dicotyledons can therefore increase in diameter indefi- 



I 



tj 





Longitudinal section of vascular bundle of sunflower stem; spiral, scalariform and pitted 

 vessels at left; next are wood fibers with oblique cross walls; in middle are cambium cells with 

 straight cross walls ; next, two sieve tubes, then phloem or blast cells. 



nitely as long as growth continues, since the cambium never com- 

 pletely passes over into permanent tissue, and extending through 

 the bundle, across the medullary ray into the adjacent bundles, 

 keeps them open. A longitudinal section of a bundle will show 

 the same arrangement of the cells, and will give an idea of the 

 length of the different elements, and show the markings of the 

 vessels and the character of the sieve tubes. 



99. Structure in cross section of perennial woody stems. 

 A cross section of a stem several years old will show the following 

 structure. In trees like the oak the chief points in the structure 

 can be seen readily with the eye or with the aid of a hand or pocket 

 lens. The character of the " bark " will depend on the age and 

 the kind of the tree. If the stem is only a few years old the bark 

 will be green and soft. This soft bark is made up of the bast 

 portion of numerous nbro-vascular bundles lying side by side, and 

 in it are the sieve tubes. On older stems the outer bark is dead 



