20 



GRASSES OF IOWA. 



by having the sclerenchyma in close proximity to the epi- 

 dermis. The peripheral bundles, including the sheath, con- 

 stitute a system of compound pillars indispensable to maintain 

 a plant in an erect position. 



The fibro-vascular bundles, in addition to the thickened lig- 

 nified elements may be divided into two parts: the soft bast or 



Fig. 13 A Cross section of corn. To the right general arrangement of bundles, 

 •a, leaf sheath ; «, where leaf originates. To the left bundles more magnified. 



phloem which consists of the sieve tubes and the companion 

 cells; second, the xylem which consists of several pitted ves- 

 sels and a spiral duct towards the center and below an inter- 

 cellular passage; this has resulted from the breaking down of 

 an old ringed vessel. 



Theodore Holm, in an interesting review of an extended 

 paper on structure of rhizomes states that, "Although the 

 function of the stolons in the Gramineee is nearly the same, 

 being at once reservoirs of nutritive matters and for the serv- 

 ice of the vegetable propagation, some differences have also 

 been observed in the interior structure. " 



It has been shown from the numerous intergradations 

 between the underground stolons and the shoots above ground, 

 that the organizations of the stolon depend upon a modification 

 of the above-ground shoot. The structure of the shoot above 

 ground is well marked by the position of the mechanical tissue, 

 which is either truly sub-epidermal or more or less distinctly 

 sub cortical, the bark being as a rule not very strongly devel- 

 oped. But there is a large series of modifications between the 

 form and those derived from such shoots as show a tendency 

 to replace the stolons. 



