THE STEM. 



29 



We have now to deal with the structure of the stem, and this 

 may be considered under the two headings of anatomy, which 

 treats of so much as can be made out with the eye alone or by 

 help of a lens, and histology, which goes into the finer details by 

 employing a compound microscope. We may commence by an 

 examination of sunflower and asparagus stems. 



Anatomy. — A good-sized example of a sunflower stem will be 

 more or less cylindrical in shape, somewhat ridged, and studded 

 with stiff hairs. If 

 an internode is cut 

 across with a sharp 

 knife, and the surface 

 smoothed by a scalpel 

 or razor, the three 

 systems of tissue can 

 be made out. There 

 is first the hair-bear- 

 ing epidermis, and 

 some little distance 

 within this a circle 

 of small oval areas, 

 separated by inter- 

 spaces. These are the 

 cut ends of the vas- 

 cular bundles, and 

 are best seen with a 

 lens. The rest of the 

 stem is made up of 

 ground-tissue, which 

 is divisible into a 

 large white central 

 part, pith or medulla ; 

 another part, cortex, 

 between the epider- 

 mis and vascular 

 bundles; and thirdly, 

 medullary rays, strips 

 of tissue running be- 

 tween the vascular 

 bundles, and con- 

 necting pith with cor- 

 tex. If now a piece 

 of the stem is halved 

 longitudinally (taking care to include a node, and to cut through 

 the attachment of a leaf), and the pith carefully scraped away 



Fig. 5.— Diagrams of Anatomy of Vegetative Organs [after 

 Prantl}. A. relation of stem and leaves ; I. leaves ; b. 

 buds; fj.p. growing-point; v.b. vascular bundles. The 

 double line on outside represents epidermis. B. origin of 

 lateral roots as seen in longitudinal section of bean ; v.c. 

 vascular cylinder ; l.r. lateral roots ; r.c. root-caps. C, D. 

 course of vascular bundles in stems of monocotyledon and 

 dicotyledon. I. leaves ; p. pith ; co. cortex. E. piece of 

 wood to show course of medullary rays ; m.r. rays ; as seen 

 in a cross-section ; m.r'. rays as seen in a radial section ; 

 m.r". rays as seen in a tangential section. 



