PLANT ORGANS AND ORGANISMS 
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been formed through the activity of the cambium: The cambium 
has developed new or secondary xylem on its inner face which has 
pushed the first formed or protoxylem toward the center of the root. 
Abnormal Structure of Dicotyl Roots. — In certain Dicotyl roots 
as Amaranthus, Jalap, Pareira, and Phytolacca after the normal 
bundle system has been formed, there then develop successive cam- 
biums outside of these bundles, producing concentric series of open 
collateral bundles. 
Histology of a Dicotyl Tuberous Root (Aconitum).- — A transverse 
section made through the tuberous root of Aconitum Napellus near 
its middle shows a cork region of one or more layers of blackish or 
brownish cells; a broad cortex of two regions, viz.: an outer narrower 
and an inner broader zone. The narrower zone consists of from eight 
to fifteen layers of cortical parenchyme cells, interspersed among 
which are numerous irregular-shaped stone cells. Separating this 
zone from the broader one is an endodermis of a single layer of tan- 
gentially elongated endodermal cells. The broader zone consists of 
about twenty layers of parenchyme cells. Next, a five- to seven- 
angled cambium, within the angles of which and frequently scattered 
along the entire cambial line, occur collateral fibro-vascular bundles. 
In the center is found a broad five- to seven-rayed pith composed of 
parenchyme cells. The parenchyme cells of the cortical regions and 
pith contain single or two- to five-compound starch grains. 
Root Tubercles 
The roots of plants of the Leguminosoe, Myricacece as well as some 
species of Aristolochiacece and of the genera Alnus and Ceanothus are 
characterized by the appearance upon them of nodule-like swellings 
called root tubercles. In the case of the Leguminosae the causative 
factor is a species of bacteria named Pseudomonas radicicola. This 
is a motile rod-shaped organism which appears widely distributed 
in soils. It is apparently attracted to the root-hairs of leguminous 
plants by a chemotactic influence probably due to the secretions 
poured out by these structures. A number of these organisms pene- 
trate the walls of the root-hairs by enzymic action. Upon entering 
the hairs they form bacterial tubes which branch and rebranch and 
