made use of in one of the preceeding keys. It may be 

 added here that the thickness of the wall of the terminal 

 internode near the tip is equally characteristic, and is 

 for the different straws as follows : — 



Thickness 



of wall 



is 



Barlev 

 Oat .'. 

 Rye . . 

 Wheat 



Diameter 

 of straw 

 6 



3- 



On this point therefore, rye and wheat may be dis- 

 tinguished from oat and barley. 



The second point is found in the number of chloren- 

 chyma spaces which may be counted in the total trans- 

 verse section. Rye has by far the fewest, namely, about 

 18 to 20, while the other species have from 30 to 36, 

 barley having 32, wheat 34 to 36, and oat 36 to 38. 

 These numbers are approximately twice the numbers of 

 bundles in the innermost series. The fact that thechlor- 

 enchyma spaces may be double instead of in pairs sep- 

 arated by a small bundle may make it necessary to re- 

 gard a single one as two, or to rely rather on a count of 

 the large bundles which lie free in the parenchyma. 

 Thus, the stem in figure 3 has 37 undies, which is, how- 

 ever, not a terminal internode. 



A third feature is the character of the lining of the 

 fistula formed y the dead parenchyma. It is thickest 

 and pavement-like in the oat (figure 17), well defined 

 in all but the wheat, in which it is loose and ilL-definecl 

 in texture, not forming a smooth lining. 



Summarizing these characters in the form of a key 

 we have the following : — 



A. Thickness of wall of fistula (terminal internode, 

 near its apex) one-fourth to one-third the total 

 diameter 



a) Collapsed pith cells forming a clear cut pave- 

 ment ; or 



b) Number of bundles in innermost series about 

 12 rye 



aa) Collapsed pith cells loose, not forming a 



pavement ; or 

 bb) Number of bundles in innermost series about 



18 wheat 



B. Thickness of wall of fistula one-sixth to one-eighth 



that of total diameter. 

 fa) Pavement formed by pith-cells thick; or 



(b) Parenchyma cells large oat 



(aa) Pavement formed by pit-cells thin; or 

 (bb) Parenchyma cells one-half the diameter of 



those in oat barley. 



The amount and disposition of the fibers (mechanical 

 tissue, bast) is correlated with the ehlorenchyma, there 

 being relatively more fibers toward the base of the (top) 

 internode, and relatively fewer toward the apex. In re- s 

 lation, however, to the area of the transverse section the-f 

 fibrous tissue is greater toward the apex, this being ap- ' 

 parently correlated with the mechanical conditions as- 

 sociated with the reduced diameter. Concerning the 

 character of the fibers we shall speak beyond. 



2. Transverse section of basal internode. If anything, 

 the .lower internodes offer less -to seize upon for pur- 

 poses of identification than the terminal. Bearing in 

 mind the difference of structure as between the upper 

 and nether parts of the internode, the middle portion 

 has been taken as a standard of comparison. Of the 

 four straws, the oat stands out clearly on account of the 

 small number of parenchyma cells, and of their large 

 size. As in the terminal internodes, the collapsed pith 

 cells make a thick pavement lining the fistula. In the 

 oat also there are only about 20 large bundles free from 



the fibre layer, while in the others the numbei 

 greater, from 24 (wheat, ryej to 28 (barley). 



Comparison of the individual tissues and of their ele- 

 ment's. Epidermis (figures 6-13 j. Of all the tissue sys- 

 tems, the epidermis lends itself most readily to identi- 



--leafj/teath 



Figure 28. — Barley. Juncture of the leaf sheath and blade 

 to show the claws and ligule. 



Figure 29. — Stomata and trichomes inner (upper) epidermis 

 of the leaf sheath of (a) barley (b) oat, (c> rye and 

 (d) wheat; also the trichomes of (e) rye and (f) wheat: 

 g., guard cells; ac, outline of accessory cells. Drawn to 

 the accompanying scale 



