30 Mr. A. Henfrey on the Structure and Habits 
large central cellular region or pith, composed of elongated ey- 
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 sometithes in contact, at others widely sepa- 
rated, not unrollable, and these are surrounded by elongated ey- 
lindrical cells with conical extremities. The wood passes insen- 
sibly into the cortieal 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 QO. 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. 
Tn 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 strue- 
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 cireumfe- 
rence, where they are inclosed by the cortical layer. The vessels 
