THE PLANT BODY 



59 



The Cambium. In the axis of typical woody plants, the cambium forms 

 a continuous cylinder, broken only by branch and leaf gaps; in some 

 herbs and subshrubs, the secondary vascular tissue of the stem is small 

 in amount; it consists of a cylindrical network of more or less discrete 

 strands — the stele is a eustele (Fig. 27B). Complete cylinders of 

 secondary vascular tissues are present in many herbs (Fig. 27A), both 

 annual and perennial; the description of herbs in general as char- 

 acterized by dissected steles is erroneous. The network of bundles rep- 

 resents a reduced and dissected vascular cylinder. Proof of this in- 

 terpretation lies in part in the cambium. In the areas between the 



B 



Fig. 27. Transverse sections of herbaceous stems showing two dicotyledonous types. 

 A, Digitalis, stele continuous; B, Artemisia, stele dissected. (From Eames and 

 MacDaniels after Sinnott and Bailey. ) 



vascular bundles, lines of tangential divisions in parenchyma cells con- 

 nect the strips of cambium in the bundles. All stages in the loss of 

 cambium between the bundles are present in related species and even 

 in the same shoot, which may have the cambium cylinder complete at 

 the base and vascular bundles free at the top, with, in between, strips 

 of vestigial cambium, which are progressively weaker toward the top. 



The evolutionary history of the cambium in the angiosperms is one 

 of reduction in activity and in area. All stages of loss are present in 

 dicotyledons; in monocotyledons, the vascular bundles typically have 

 no cambium, but, in many genera, some of the bundles have a weak 



