THE STEM 203 
EXCEPTIONAL Types or DicotyL STEMS 
In a number of Dicotyledons and Gymnosperms, the second- 
ary 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 stems of Phytolacca, etc., there first arises a ring of primary 
bundles with broad, loose medullary-rays. Then the stem 
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Fic. 132.—Transverse section of the herbaceous dicotyledonous stem of Solanum 
tuberosum (Fam. Solanacez) showing bicollateral bundles with internal phloem. 
X 75. ep, epidermis; co, cortex; ph, external phloem; ca, cambium; x, xylem; 7, 
internal phloem; si, sieve tissue; m, medulla. Both phloem regions contain primary 
phloem. 
cambium ceases its activity and, outside the bast of the bundles 
already formed, in the pericycle 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, the cambium closes up. Still 
another cambium ring arises without this which lays down 
another circle of bundles and, in a single season quite a number of 
these are found successively arranged in concentric fashion. 
