264 STRUCTUEE OF STEMS AND ROOTS 



seen (425) , there is a cambium or growing layer in every 

 open collateral bundle just between the xylem and phloem. 

 Each spring the cells of this layer divide many times and 

 form new cells both inside and outside the cambium ring. 

 Those formed inside become thick walled and are xylem. 

 Those formed to the outside of the ring are gradually 

 changed into phloem. The crowding of the cells within 

 the cambium ring causes the ring itself to enlarge its 



406. Enlargement of a single concentric bundle from Fig. 405. 



circumference and move outward by this growth. To 

 study secondary thickening : Cut thin cross -sections of 

 basswood stems of different ages (one to three years old). 

 Stain and mount. Examine with low power and sketch 

 the arrangement of bundles in the oldest and youngest. 

 Note the effect of growth on the medullary rays. Test 

 them with iodine for starch. Now with the high power 

 study the peculiar character of the bast tissue. Note the 

 abundance of fibrous tissue found all through it. Draw 

 a single bundle from the stem one year old, carefully 



