372 Cooke and Schively on Observations on the 



transferred to the hard bast. It is this great development of 

 bast fibres that gives the stem its stiff inelastic nature. It 

 probably was in order to form an external protection to the 

 enlarged phloem region, that such a great development of 

 hard bast occurred. The whole bundle arrangement is evi- 

 dently intended to allow the free passage of sap out into 

 the stem at all places through its course, and to insure the 

 abundant and widespread storage of food all along the stem. 



The Tuber. 



Sections made through a large mature tuber present an 

 extremely irregular and complicated aspect. Very striking 

 is the great development through the parenchyma of thick- 

 ened cells identical with the "hard bast" of the aerial stem 

 bundles. These cells evidently are not a part of the true 

 bundle system, and must originate in periblem rather than 

 in plerome tissue.^ The bundles show a most confused 

 and irregular arrangement, running in all directions and 

 planes apparently. The phloem of a bundle is greatly in ex- 

 cess of the xylem, and shows a tendency to spread out and 

 lie in separate patches, while the xylem of each bundle seems 

 always concentrated in a single area. Many of the bundles 

 show an internal duplication with reversed order, phloem, 

 xylem, xylem, phloem, succeeding each other from without 

 inwards. In such a case, an area of thickened yellow 

 stained cells often intervenes between the two xylem areas, 

 and the whole may have resulted from fusion of two bun- 

 dles. An internal phloem is almost always present, often 

 in excess of the outer phloem mass. 



A wide ring of the thickened, so-called "hard bast" cells 

 lies below the narrow cortex, being quite regularly inter- 

 rupted by thin-walled parenchyma cells passing in toward 



' These rhizome sections were double-stained with Kleinenberg's 

 haematoxyHn and Bismarck brown, the thickening then taking on a 

 yellow or brown stain. 



