PLANT ORGANS AND ORGANISMS 145 



EXCEPTIONAL TYPES OF DICOTYL STEMS 

 In a number of Dicotyledons and Gymnosperms, the secondary 



growth in thickness of the stem and frequently of the root differs from 



that which is found in the vast majority of species and so is called 



exceptional or anomalous. 



InPhytolacca, etc., there first arises a ring of primary bundles with 



broad loose medullary rays. Then the stem cambium ceases its 



PIG. 72. White birch (Betula populifolia). Portion of a branch showing the 

 prominent lenticels. (Gager.) 



activity, and, outside the bast of the bundles already formed in the 

 pericambium or tissue developing from it, a new cambium starts 

 to lay down another ring of bundles in rather irregular fashion. 

 Then after developing a wavy ring of bundles and connecting tissue 

 that cambium closes up. Still another cambium ring arises without 

 this, arid in a single season quite a number of these are found succes- 

 sively arranged in concentric fashion. 



In Gelsemium, species of Solanacece, Combretacece, Cucurbitacece, 

 etc., there arises a cambium on the inner face of the xylem which 



