4i5 
of Dioscorea prehensilis. 
invariably blocked by callus, which fact probably points to 
the conclusion that the cause Is physiological rather than 
mechanical. 
The breaking is not effected at any one particular point ; 
there is rather an ill-defined region where the transverse 
severance may occur. 
Sections taken above and below the rupture, where the 
apical region was preserved, exhibited no anatomical charac- 
ters such as are found in the separation-layer of a leaf. 
The first sign of breaking in a young spine (PL XIX, 
Fig. i C) is a slight brown annular discoloration in the region 
indicated by the line X- F, and sections show that here the 
softer tissues are withering. The portions of the root nearer 
the apex appear quite healthy, hence it is obvious that the 
rupturing is not caused by the mere dying of the tissues from 
the apex backwards. 
In a more developed spine the discoloration-zone is seen 
to be still more marked and the separation is almost com- 
plete, connexion only being maintained by shreds of cortex 
(Fig. i D). 
Besides these large spine-bearing roots the tuber also 
possesses much smaller roots, having numerous lateral root- 
lets which are clearly absorbent in function. These secondary 
roots, after dying off, also leave behind a small hard spine 
similar to those of the larger roots; a fact which has already 
been pointed out by Dr. Scott L Indeed, it may be stated at 
once that there is no essential difference either in the morpho- 
logy or anatomy between the two varieties of roots, the 
dissimilarity being only in the size. 
Structure. 
Our knowledge of the anatomy of the roots of the Diosco- 
reaceae is based chiefly on the researches of Bucherer 2 who, 
in his work on the anatomy of this natural order, confines 
1 Loc. cit„, p. 329. 
2 Emil Bucherer : Beitrage zur Morphologie und Anatomie der Dioscoreaceen, 
Bib. Bot., Heft 16, iii, 1889. 
