30 Mr. A. Henfrey on the Structure and Habits 



large central cellular region or pith, composed of elongated cy- 

 lindrical cells ; these pass gradually, without the intervention of 

 a medullary sheath, into the woody region composed of a num- 

 ber of fibro- vascular bundles arranged in a circle and forming a 

 continuous envelope to the pith, no medullary rays existing. 

 The wood, which is very deficient in quantity compared with the 

 pith and cortical layer, contains spiral fibrous vessels, the turns 

 of the spirals being sometimes in contact, at others widely sepa- 

 rated, not unrollable, and these are surrounded by elongated cy- 

 lindrical cells with conical extremities. The wood passes insen- 

 sibly into the cortical parenchyma which forms a very broad 

 region, composed of cells resembling those of the pith, and it is 

 clothed externally by a layer of epidermis, the cells of which 

 have the form of elongated prisms. Stomates appear to be very 

 rare ; I observed none in a number of portions of epidermis of 

 O. rapum taken from all parts of the stem, but the cells were 

 often filled with a brown resinous secretion. In O. minor this 

 secretion is less abundant, and I observed a few stomates here 

 and there. In both species the epidermis is clothed with nume- 

 rous capitate glandular hairs ; these consist of filaments formed 

 of three or four cells attached end to end and gradually dimi- 

 nishing in diameter upward, terminated above by a globular 

 body consisting of one, two or three cells, filled with a resinous 

 secretion. 



In full-grown specimens the lower part of the stem is enlarged 

 into a bulbous expansion which appears to me to be a true tuber. 

 It presents a central parenchymatous region, which by its en- 

 largement forces the fibro-vascular bundles apart, so that they 

 lie irregularly toward the periphery, beneath the cortical region 

 continuous with that of the upper part of the stem. The vas- 

 cular structures in the tuber consist, not of spiral vessels like 

 those of the stem, but of longish cells, which from their varying 

 direction have not been thrown into long ducts like the vessels 

 above, by the absorption of their contiguous ends, but retain 

 their cellular form, while the deposition of secondary layers has 

 gone on to the conversion of the spiral into the reticulated struc- 

 ture. 



The stem and upper part of the tuber are furnished with fleshy 

 scales which are composed of cellular tissue, and have fibro-vas- 

 cular bundles running into them from the woody zone. 



The roots bear some resemblance to those of Monocotyledons. 

 They present a central vascular region composed of about four 

 bundles disposed so that the vessels present a cross in the trans- 

 verse section, but the woody cells forming the remainder of the 

 bundles are blended into a mass, well-defined at the circumfe- 

 rence, where they are inclosed by the cortical layer. The vessels 



