ADVENTITIOUS OR SECONDARY ROOTS. 129 
ultimately having a similar structure to that of a monocotyle- 
donous stem. Where they break through they are surrounded at 
the base by a kind of sheath or collar called a coleorhiza (fig. 251, 
co). They also grow by additions within their extremities like 
true roots, and are terminated like them by a root-cap or pileo- 
rhiza. In the adventitious aerial roots of the Screw-pine (jig. 
199, 2), and some other plants, the pileorhiza may be well seen in 
the form of a cap-like covering at the extremity of each root 
or branch of aroot. The pileorhiza of a monocotyledonous root, 
like that of a true root, is commonly thrown off as development 
takes place behind it; but in certain aquatic plants, as in the 
Duckweed (jig. 252), it is persistent, and appears in the form 
of along sheath over the end of the root; and is continually 
pushed onwards by the development of the cells within the apex. 
ETE Jol Fic. 252. Fig. 253. 
Fig. 251. Germinating embryo of the Oat. 1. Rootlets, each with a sheath 
(coleorhiza), co, at its base. c. Cotyledon, g. Young stem.— Fig. 252. 
Magnified plants of the Lesser Duckweed (Lemna minor), with the roots 
covered by a long root-cap (pileorhiza). Fig. 253. Lower part of the stem 
and root of the common Stock, 7. The tap-root with its branches. c. The 
base of the root or point of union between the stem and root which was 
formerly termed the neck. ¢. Thestem. j, 7. Leaves. 6,0. Buds in process 
of development into branches, 
The adventitious roots of Dicotyledons arise in a somewhat 
similar manner to those of Monocotyledons, making their first 
appearance as little conical bodies formed from the substance of 
the pericambium, and ultimately breaking through the bark and 
appearing on the surface. They also grow by additions within 
their extremities, and each is protected by a pileorhiza, and has 
at its base a coleorhiza. They have under ordinary circum- 
stances a similar structure to that of true roots. 
K 
