THE STRUCTURE AND GROWTH OF STEMS 171 



many road-side weeds also have pithy stems of this type. 

 Of hollow stems of this type that of the jewel weed or 

 balsam (Impatiens) is an excellent one for observation. 

 It is abundant in damp woods after the middle of May. 

 The stem of this plant is translucent; that is, when you 

 hold it between you and the light, the light shines through. 

 This permits you to see the vascular bundles separately, 

 somewhat as you see the bones in an X-ray picture of the 

 hand. 



E. The Scattered Arrangement. — Reference was made 

 to the scattered arrangement of vascular bundles on 

 page 82. It was said of the corn stem that the vascular 

 bundles lie, in a cross section of it, "like islands in a sea of 

 softer tissue." That softer tissue is a sort of combination 

 of cortex and pith. In the cylindrical arrangement we have 

 the bundles forming a hollow cyUnder; pith is on the 

 inside of it and cortex on the outside. But in the scattered 

 arrangement we have no such dividing cylinder ; the soft 

 cells form a continuous mass, the bundles being scattered 

 through it. There is no such thing as dividing this mass 

 of soft cells into cortex and pith ; they are all about aUke ; 

 cortex and pith are undifferentiated. 



In scattered bundles there is no cambium. Since there 

 is no cambium, there is no forming of new cells in the 

 bundles. With respect to cambium, bundles which lack 

 it are called closed; bundles which possess it are called 

 open. Closed bundles have no growth save primary 

 growth; open bundles are "open" for secondary growth. 

 This possession or lack of cambium in the vascular bundles 

 greatly affects the general form and habit of a plant. The 

 lack of it generally means that there is no gradual enlarge- 



