514 MISS ISABEL MCCLATCHIE ON 
off before division into lobes takes place. In this, as in similar cases, the 
division into lobes is preceded by the development of cork, which in sections 
from above downwards seems first to arise in the cortex and to pass inwards 
to the pith, thus dividing the root into segments. 
In text-fig. 20 а-у are shown a root in which a number of lateral rootlets 
are given off just above the wound that removed the apex and slanted towards 
one side (that from which the rootlets developed). The root itself is divided 
into three portions corresponding to the three projections of the plerome ; 
two persist and develop root-apices. 
In many cases where the injury to the apex is very severe and a consider- 
able portion of the tissues is removed, a number of rootlets develop on the 
proximal side of the wound. The plerome itself develops no new apices, and 
the root appears to divide up into numerous rootlets. Text-fig. 20 h-g show 
this type of wound reaction resulting from a severe oblique decapitation. 
SUMMARY. 
The primary root-system of /mpatiens Roylei, Walp., is of the normal 
dicotyledonous type, consisting of tap-root and lateral roots. This root- 
system remains shallow, and begins to be reinforced by а secondary 
root-system of an adventitious nature when the seedlings are about twenty 
centimetres high and have initiated a fistular cavity. These earlier adven- 
titious roots arise, like the primary lateral roots, from the protoxylem poles, 
and in some cases seem to develop in definite acropetal succession, Subse- 
quent to this the appearance of roots is irregular and depends to a certain 
extent upon the peculiarities of the individual plant, but there are certain 
conditions which always lead to their development. 
Should the main axis become inclined or curved, roots will develop from 
the convex or under surface. This feature is most marked in the hypocotyl 
and at the bent nodes ; the roots decrease in frequency from below upwards, 
Roots may be produced from robust lateral branches, and here, again, it is 
from the convex surface of the basal swellings that they develop. 
A damp atmosphere is favourable to root-growth, but it is not the primary 
cause of their development. Soil contact is also a potent factor in stimulating 
their development. 
Adventitious roots, which are usually abortive, invariably arise as the 
result of. wounding, most probably as a response to the interference with 
the translocation of water and food substances. Their appearance in plants 
with slit internodes is due to a similar stimulus. 
In these two cases the roots develop immediately above the wound or slit 
whether it be on the concave, convex, or vertical surface of the stem. 
The position of the majority of aerial roots makes them necessarily abortive. 
