18 



GRASSES OF IOWA, 



Fig 12. Section 

 of culm of Wild 

 Rice (Zizania aqii- 

 atica L ) showing 

 fibro- V ascu 1 a r 

 bundles and 

 chambered pith. 

 (King.) 



the main axis. The presence of this prophyllum always indi- 

 cates the presence of a branch, although the branch may be 

 very much shortened, as in the case of the true floral axis 

 where this prophyllum is the palea. 



Mlmite anatomy of stem. — A cross section of 

 a culm from which the leaf sheath has been 

 removed will show first an epidermal layer com- 

 posed of th'ck walled cells, isodiametric in 

 Zizania aquatica, but longer than wide in Bromus 

 mollis. Variations o3cur in other genera. In a 

 longitudinal section the epidermal cells are 

 longer than broad. The epidermis is unbroken 

 except for the stomata, which are not as fre- 

 quent as in the leaf and sheath. The stomata 

 connect with the loose spoogy parenchyma. In 

 Zizania and Bromus,' Zea and other grasses the 

 epidermis is followed by sclerenchyma sheath. 

 This sheath varies in thickness in different 

 grasses, being particularly well developed in 

 Zizania and Zea. In some grasses like Zizania 

 aquatica there are two sclerenchyma sheaths, 

 one immediately under the epidermis, a second layer below 

 the spongy parenchyma. Sclerenchyma is also found in con- 

 nection with the bundles. 



The fibro-vascular bundle is especially prominent. In pith- 

 less culms the fibro-vascular bundles are arranged, usually, in 

 two circles; one occurs on the inside of the sclarenchyma 

 ring, the other on the outside of it. 



In species with pith, like Zizania, a first row of bundles 

 occurs on the outer edge of the second sclerenchyma sheath. 

 A second row of bundles occurs just underneath or a little 

 below the sclerenchymi sheath. Th3 remaining bundles are 

 arra J ged in indistinct circles. Hackel says: '-The fibro-vas- 

 cular bundles run parallel in the internodes; the superficial 

 ones join those of the lower internodes directly, the others 

 take the form of a shallow arch bendiag towards the center 

 (in culms with pith), where they pass through several inter- 

 nodes, and finally bend outwards to join the super dcial ones. 

 In the nodes the bundles cross and interlace and by means of 

 small and short cross-bundles, which pass from the axillary 

 shoots or buds towards the center. In this way arise the dia- 



