STEMS OF DICOTYLEDONS 157 



infer from this as to the office of the ducts ? How does 

 this conclusion compare with your observations on the 

 vascular bundles of monocotyledonous stems ? Notice 

 that the dicotyledon differs from monocotyledonous stems 

 in having the pith all gathered in a narrow cylinder in the 

 center, and the vascular tissue arranged in one or more 

 concentric layers around it, according to the age of the 

 stem. In general, dicotyl stems may be said to include 

 four regions; ist, the epidermis or bark, e (Fig. 300); 

 2d, the cortex, c, made up of the cambium and bast, with 

 certain other tissues ; 3d, the vascular cylinder, or woody 

 portion w, made up of concentric rings each representing 

 a year's growth ; and 4th, the pith /, medulla, or paren- 

 chyma, as it is variously termed by botanists, 



222. Medullary Rays. Observe the whitish silvery lines 

 that radiate in every direction from the center, like the 

 spokes of a wheel from the hub. These are the medullary 

 rays and consist of threads of pith that serve as lines of 

 communication between the "parent tissue" and the grow- 

 ing cambium layer. In old stems the central pith fre- 

 quently disappears and its office is filled by the medullary 

 rays, which become quite conspicuous. 



223. The Rings, into which the vascular cylinder is 

 divided, mark the yearly additions to the growth of the 

 stem, which increases by the constant addition of fibro- 

 vascular bundles from the outside ; hence such stems are 

 called exogens or "outside growers." 



224. The Structure of the Fibro- 

 vascular Bundles is somewhat com- 

 plicated and can not be studied to 

 advantage without the aid of a com- 

 pound microscope, but a little atten- 

 tion to the diagrams will make it 



. 11- -LI T-L f i 302. Transverse section 



intelligible. The inner part of each of vascu]ar bundle from 

 bundle (i.e., the part toward the axis) stem of a dicotyledon: /, 



j r j /-i i bast; c, cambium; v, ducts, 



is made up of woody fibers shown at w> wood ce n s . 



