LABORATORY STUDIES 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 window light, shows 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. 



(6) 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) pith, (b) hollow cylinder of wood, and (c) bark. 

 Strictly the bark includes the outer ends of the elongated areas noted 

 under (ft), 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 



(6) The scattered darker parts which 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, 



