THE ROOT-SYSTEM OF IMPATIENS ROYLEI. 515 
They remain short and stunted, and are soon protected by a layer of cork. 
A marked decrease in diameter, about, one-third of the distance from the tip, 
is a characteristic feature of such abortive roots. 
Should these roots, however, be surrounded by soil, they will, like the roots 
developed from the lower part of the hypocotyl, produce lateral roots freely 
and will grow to a considerable length. 
In the primary root-system the tap-root is tetrarch with a solid xylem ; 
it may occasionally develop a considerable amount of secondary wood. The 
lateral roots are generally pentarch or tetrarch, but occasionally triarch. In 
both main and lateral roots the whole of the cortex cells external to the 
endodermis become divided and suberised, and in both the vessels become 
closed by tyloses. 
The adventitious roots are polyarch with feeble xylem development and a 
large pith. Tannin cells are common in the cortex, and also appear, though 
less frequently, in the pith. Raphid sacs are present in the pith and less 
frequently in the cortex. 
Rootlets rarely arise except from subterranean adventitious roots and from 
roots which have suffered a severe lateral injury or in which the apex has 
been removed. In the latter case a little rosette of rootlets is often formed. 
The rootlets are usually tetrarch, but they vary considerably and may have 
from three to twelve xylem poles. 
Roots sometimes appear to dichotomise, or to develop three or four apices. 
This is merely the result of injury to the root-tip and the consequent develop- 
ment of new root-apices from the torn plerome. 
In conclusion, I should like to express my thanks to Mr. H. S. Holden, at 
whose suggestion the work was undertaken, for his constant help and advice, 
and also to Professor Carr, in whose department the work has been done and 
who granted every facility for its execution. 
EXPLANATION OF THE PLATES. 
PLATE 37. 
= 
5 
d 
: | Young seedlings showing the primary root-system and the varying length of the 
hypocotyl. 
4. Seedling with lateral roots of hypocotyledonary origin. 
Seedling with primary lateral roots arranged in a whorl on the hypocotyl. 
6. Primary root-system showing abrupt change in diameter а the junction of the 
hypocotyl and the tap-root; note that the tap-root is short and the root-system 
shallow. 
d Primary root-systems showing а more gradual change in diameter and the ultimate 
8. \ narrowing of the tap-root. 
