LABORATORY ST[IDIE:S OF THE STEM 47 



These considerations lead us at once to examine the internal struc- 

 ture. We shall expect to find out whether the internal construction 

 answers to the uses of the stem or not. 



1. A comparison of dicotyledonous and monocotyledonous stems. — 

 Begonia {dicotyledon), Asparagus (^monocotyledon). 



(1) Even a naked-eye examination of the cross sections, held up 

 side, by side to the wdndow light, show^s marked differences. Consider 

 carefully wherein they are alike and wherein dissimilar, and write a 

 comparative account of the cross sections as you see them, 



(2) Place the Begonia section under the highest power of the dis- 

 secting microscope. Notice the following points : — 



(a) The central space is filled with a more or less irregular and 



indistinct network, in which some meshes {cells) of tolerably 

 regular form may be made out. 



(b) Outside of this is an interrupted circle of somewhat wedge- 



shaped, denser spots, nearer the circumference than the center 

 of the section. 



(c) Exterior to these is a region filled by a network of large cells. 



Toward the margin, however, the cells become gradually 

 smaller. 



The outermost layer of cells, which may not be distinguishable, is 

 of a distinct nature, and forms the epidermis. 



The three regions thus noted are characteristic of dicotyledonous 

 stems. They are {a) the pith, {b) the ring of wood, and (c) the bark. 

 Strictly the bark inckides the outer ends of tlie elongated areas noted 

 under {b), and only the inner half or two-thirds is wood. (The lens 

 will probably show the division line.) In this fleshy herbaceous stem 

 the wood does not form a complete ring in the cross section, it will be 

 noticed. The Lilac, soon to be studied, will show an apparent differ- 

 ence in this respect. 



Draw a sector of the cross section, showing the character of the 

 three regions (x 5 — 10). 



(3) Examine in the same manner the section of Asparagus. 

 Note : — {a) The large cells composing by far the greater part of 



the section. They are replaced by cells of a different char- 

 acter in two instances ; namely, in 



{b) The scattered darker parts w^hich much resemble the denser 

 areas in Begonia ; and in 



(c) A distinct dense ring, not far from the edge of the section. 

 Finally there is 



{d) The outermost zone, composed of round cells of uniform size 

 (the epidermis). 



The monocotyledonous stem has no separate region of wood includ- 

 ing pith and surrounded by bark, such as one finds in dicotyledons. 

 A cylinder of firm tissue (c), giving a degree of rigidity to the stem, 



