Tissztes in Certain Monocotyledons . 43 
root independently of any further communication with the 
pericycle. 
It is not probable that all lateral roots form the starting- 
points for secondary growth. In some cases they showed 
no secondary bundles at the base (though there was some 
radially arranged parenchyma) and no signs of cambium. 
The connection between the internal and external tissues 
is established partly on a large scale, where the two zones 
are continuous, through large gaps in the endodermis of the 
transitional region ; partly on a small scale by local connec- 
tions. We have several times seen a single secondary bundle 
passing through the endodermis, and in one case we found 
a horizontal strand of tracheides connecting the primary 
xylem of the cylinder with the secondary tissues which had 
been formed in the cortex. We can confirm Strasburger’s 
statement that vessels only occur in \}s\e primary xylem-groups 
of the roots. The secondary bundles are quite like those of 
the stem, and their water-conducting elements are invariably 
tracheides. 
It appears, then, that in most of the roots investigated by 
us the formation of secondary tissues starts from the insertion 
of the rootlets, and at first serves to establish additional 
channels of conduction between the branch and its parent 
organ. Subsequently the process of new formation thus 
initiated extends to all parts of the root. An acropetal 
formation of secondary tissues, starting from the base of the 
adventitious root itself, also occurs, but does not extend far, 
and serves to establish the connecting link between the scond- 
ary tissues of the root and those of the stem. 
It may be mentioned that the cambium in the root often 
forms several layers of secondary cortex on its outer face. 
Periderm is regularly formed in these roots, sometimes from 
the layer next within the exodermis, sometimes from a deeper 
layer of the cortex. 
Our results may be summed up as follows : — 
(1) In the adventitious roots of Dracaena fragrans and 
D. Draco , the secondary growth in thickness starts from 
