PLANT ORGANS AND ORGANISMS 
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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. 
In Phytolacca , etc., there first arises a ring of primary bundles with 
broad loose medullary rays. Then the stem cambium ceases its 
Fig. 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, and 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 
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