the Sivcct Potato. 209 



of the tubular piece, both passing down into the soil and 

 becoming functional. 



Histology of Normal and of Ring-fasciated Shoots. — 

 For a better understanding of the cross-section of the ring- 

 fasciated stem, we may first review the structure of the normal 

 stem (Fig. 2). There is a cutinized epidermis bearing stomata, 

 nectar glands ^ and pedicellate hairs ; internal to it is the 

 green cortex, bounded on its inner face by a distinct starch- 

 bearing bundle-sheath. Within this is a ring of bast fibres, 

 then true phlcem, cambium, xylem with many confused and 

 insignificant medullary rays, and an unbroken pith. Numerous 

 patches of internal phlcem that constitute a bicollateral bundle 

 system lie between the xylem and the pith, as is frequently 

 the case in Convolvulaceae and allied orders. Milk canals 

 are numerous in pith and cortex. 



The ring-fasciated stem shows a doubling of the above 

 structure, in that the internal cavity is surrounded by epidermis, 

 cortex, and a fibro-vascular ring in addition to those forming 

 the normal external wall of the stem. We find, therefore, in 

 cross-section of the anomalous stem, the following zones, 

 passing from without inward (Fig. 4) : epidermis {Ep.) with 

 cuticle, nectar-glands and pedicellate hairs, cortex {Co.), 

 bundle-sheath {B. S.), bast fibre ring {B. F.), normal phloem 

 (Ph.), cambium {Ca.), xylem {Xy.), internal phloem (/. Ph.), 



^ Poulsen (18) describes nectar glands situated in pits at the top of the petiole 

 of " Batatas edulis'^ (= Ipomcea Batatas Poir. ), but makes no mention of simi- 

 lar, though less specialized, glandular hairs which occur on the lamina and on the 

 stem of the plant. They are identical in structure and development with those 

 described and figured by Ewart (6) for Ipomcea patiiadata. They are so plenti- 

 ful on the apex of the stem and on the young leaf rudiments as to cover fully 

 seven-eighths of the surface of those parts. As the parts grow older the glandu- 

 lar hairs become brown and shrivelled, and most of them fall off. On mature leaves 

 they were found beset with a fungus, as also were the petiolar nectaries ; the 

 parasite was evidently feeding on the nectar. Poulsen says ants and aphides are 

 likewise attracted to the sweet potato tips ; the former doubtless serve as a protec- 

 tive bodyguard against caterpillars, grasshoppers, snails, and the like. 



14 



