36 Jackson. — Anatomical Structure of the Roots of Barley . 
laterals ; the branched roots are thin and give off laterals when only an inch 
or two long. 
2. Abundant root-hairs exist all along the ‘ unbranched ’ roots, but only 
behind the growing points of the branched roots. 
3. The central region of the stele of an ‘ unbranched ’ root is traversed 
by four to six ducts with comparatively thin walls, while in the branched 
root there is only one central axile duct bounded by thick-walled cells. 
4. In an ‘ unbranched ’ root the endodermis, pericycle, and all the 
outer tissues of the stele, with the exception of the xylem vessels, are thin- 
walled ; in a branched root all the stelar tissues, especially the endodermis, 
are very much thickened. 
5. The number of xylem vessels varies from twelve to sixteen in the 
‘ unbranched ’ root, but from six to eight in the branched root. 
These differences are represented diagrammatically in Fig. 12. 
Functions of the ‘ unbranched ’ roots. Branched roots having identically 
the same structure as that of the primary branched roots are sometimes 
found springing from one of the nodes above the grain. Such a root must 
be of adventitious origin, and it is therefore certain that the differences in 
structure existing between the ‘ unbranched ’ and the branched roots are 
not due merely to the fact that the former are adventitious while the 
majority of the latter are present in the embryo and spring from the grain. 
It seems probable that the ‘ unbranched ’ roots have some special function 
for which they are adapted by their structural peculiarities. 
It might be suggested that they are of the nature of contractile roots, 
and serve to pull the plant down to a more favourable position in the soil. 
But contractile roots generally show a very distinct wrinkled surface caused 
by the cortical cells of the roots becoming, by their turgescence, more 
stretched transversely than longitudinally, and such an appearance is not 
found in the ‘ unbranched ’ roots. Further, these roots are always present 
in water cultures where contractile roots would certainly not be needed. 
Another suggestion is that they are a special system developed to 
provide extra support for the plant when the parts above ground are 
increasing in weight. The 4 unbranched ’ roots spread out considerably 
and form a network just beneath the soil surface, so that they are in 
a position to give good support to the plant. They are then physiologically 
comparable with the stilt- or buttress-roots occurring in Pandanus , Rhizo- 
phora , &c., and still more comparable with the adventitious roots which 
arise from the lower nodes of the stem of Zea Mais. Haberlandt, 1 following 
Warming’s description of the stilt-roots of Rhizophora Mangle , states that 
they are of a stem-like structure. The middle of each root is qccupied by 
a large pith surrounded by alternating groups of xylem and phloem, while 
thick-walled mechanical cells are found on the medullary side of the water- 
1 Haberlandt : loc. cit., p. 188. 
