164 
Journal of Agricultural Research 
Vol. XXVII, No. 3 
one another by a single large pore. The lumen of the larger cells becomes 
filled with tyloses which are most prominent in the mature root. 
The storage parenchyma is of two types: (a) Normal bundle paren¬ 
chyma which, like the xylem and the phloem, is a product of the cambium; 
(b) interstitial parenchyma—a filler between the groups of bundles. The 
interstitial parenchyma can be considered the direct progeny of the cells 
of the parenchymatous sheath of the young rootlet. It forms irregular 
areas and even broad zones, which are distinguishable from the surround¬ 
ing tissue by their lighter color. The cells composing it are commonly 
irregular, elongated, and poor in starch. The cells of the bundle paren¬ 
chyma are polyhedral and fairly uniform; they are very rich in starch 
and possess a well developed nucleus. 
Fig. 6 .—Cross section of cambium of large fleshy root. Notice the formation of a secondary cambium to 
the inside of the group of xylem elements. 
TISSUE BREAKDOWN IN STORAGE 
Harter and his coworkers (2) observed that certain sweet potato roots 
which had been in storage for a number of weeks would show symptoms 
of internal breakdown. An examination of the affected areas showed 
that the cells were partly destroyed and that their starch content was 
greatly reduced. In general, however, the tissues exhibited no chemical 
changes which would indicate far-reaching pathological disturbances. 
From the study of the structure of the normal tuber, it will be recalled 
that the groups of vascular tissue are separated by undulating bands or 
areas of parenchyma. When tubers in the initial stage of breakdown are 
examined, it becomes evident at once that the first pathological disturb¬ 
ance has taken place in the interstitial parenchyma. The cells of this 
tissue are large, irregular, and poor in starch; vascular elements and latex 
tubes are entirely wanting. When breakdown occurs, the cells of the 
affected region become at first more or less dehydrated; this is indicated 
by their infiltration with air, giving them a pure white appearance 
