498 Dale . — Origin , Development , and Morphological 
half an inch long and about an eighth wide. They were 
covered by a rough, almost scaly epidermis. 
It is evident from the foregoing observations that the 
axillary tubers of Dioscorea sativa are of the morphological 
.nature of stems bearing buds which are both axillary and 
adventitious, and roots which are adventitious. Apparently 
the underground tubers in this species are of the same 
morphological value. When developed below the ground, 
the roots in the tuber grow out and become functional, 
whereas in the aerial tuber, even if this be planted, they 
never appear outside the epidermis or periderm. This may 
be due to the fact that in large tubers the external tissues 
are thick and corky, so that the root may not be able to 
penetrate them. 
Concerning the morphological value of the different kinds 
of tubers in various species of Dioscorea there is diversity of 
opinion. 
De Bary 1 places the underground tubers in three cate- 
gories : — 
(1) Tuberous swollen roots, e. g. Dioscorea Batatas. 
(2) Rhizomes with scaly leaves and composed of many 
internodes, e. g. Dioscorea villosa. 
(3) Leafless tubers, resulting from the swelling of the first 
epicotyledonary internode of the seedling, e. g. Tamils com- 
munis , T. polycarpus , Testudinaria , and many species of 
Dioscorea. 
There seems to be a general consensus of opinion that the 
underground tubers of D . Batatas are true roots 2 . According 
to Royer 3 the perennial part of D. Batatas is a small almost 
globular body about the size of a hazel nut, situated at the 
top of, but distinct from, the tuberous root. Each year this 
perennial organ, which is morphologically a stem, produces 
a twining stem and a tuberous root, and it is itself marked 
1 De Bary, Comp. Anat. of the Phanerogams and Ferns, Eng. edit., p.622 (’84). 
2 De Bary, 1. c. Engel and Prantl, vol. ii, 5, p. 131 (’88). 
3 Royer, Le tubercle de l’lgname est une racme, mais non pas un rhizome. 
Bull, de la Soc. Bot. de France, vol. xxx, p. 225 (’83). 
