Jan. 19, 1924 
Anatomy of the Sweet Potato Root 
165 
whereby they stand out strikingly from the surrounding tissue (PI. 4, A). 
This parenchyma gradually acquires a spongy texture as the obliteration of 
cells progresses, leading finally to the formation of small, polyhedral 
chambers which are lined by the remnants of the destroyed tissue. (PL 
4, A, B, C, D.) 
This process of progressive obliteration begins simultaneously at many 
points, spreading slowly from the center of the fleshy root along the ver¬ 
tical axis and in extreme cases involving the entire root. During the 
formation of these cavities, transverse zones of tissue may remain in 
certain places, forming diaphragms which break the continuity of the 
longitudinal hollows (PL 4, A). The diaphragms appear as thin, trans¬ 
lucent membranes from one to several cell layers thick. The walls of 
the cells are for the most part still cellulose, but some of the elements 
become lignified. In severely affected fleshy roots the diaphragms also 
break down, so that long, continuous air passages traverse the entire 
root. 
In advance stages of breakdown, obliteration of the tissue spreads to 
the parenchyma of the individual bundles, resulting in the denudation 
and complete isolation of numerous vascular strands. 
Except for the lignification^of individual cells, the tissue lining the 
cavities undergoes no change in the composition of its cell walls. Oc¬ 
casionally, however, roots show a pronounced discoloration of the cavi¬ 
ties, and a browning and partial lignification of the tissue bordering the 
hollows. The discoloration may remain more or less confined to the 
lining of the cavities, or it may spread so as to involve several layers. 
At such a stage, a section through a root shows a picture not unlike that 
produced by certain dry rots. The apparent sterility of these cavities 
and the absence of external injuries, however, suggest no complications 
resulting from parasitic infections, and the browning of the tissues must 
be considered, for the present at least, as an advanced stage of the 
original breakdown. 
The hollows which result from the breakdown of the fleshy root tissue 
resemble in their appearance the lysigenous air spaces found in the stems, 
leaves, and roots of grasses; sedges, and a host of other plants. The 
formation of these air passages, according to de Bary (1, p. 216), 
begins by those cells which do not follow the growth of the tissue surrounding them, 
becoming at first separated from one another so as to form schizogenous cavities which 
gradually increase in size. The cells of the tissue thus broken up then gradually 
lose their protoplasm, dry up and coalesce as flaky masses which are attached to the 
wall of the cavity. In other cases, the cells first lose their protoplasm, the membranes 
become apparently thinner, and finally rupture by the extension of the surrounding 
tissue. 
In all these cases the formation of air cavities is a normal phenomenon 
in the growth and differentiation of the organs of certain plants. The 
cavities themselves are partly the result of unequal growth of the tissues. 
Similar agencies are at work when, under abnormal climatic conditions, 
organs such as Irish potato tubers show extensive internal hollows. 
These cavities, however, do not develop after the tuber is mature and 
placed in storage. 
While the cavities occurring in the sweet potato have much in common, 
as far as appearances go, with those resulting in other plants from normal 
growth processes, and though the mechanism of their formation might 
be the same, or similar, if we consider typical lysigenous cavities, we have 
still to determine what factors are responsible for such changes in the 
