Jackson. — Anatomical Structure of the Roots of Barley. 37 
conducting tissue. For the adventitious roots of Zea Mats Haberlandt 
describes a fibro vascular cylinder enclosing a wide core of pith surrounded 
by alternating groups of xylem and phloem. Very wide vessels disposed 
in a ring are conspicuous in the xylem groups, and the endodermis is thick- 
walled. Further, there is a zone of thick-walled parenchyma found in the 
cortex just within the piliferous layer. The ‘ unbranched ’ roots of barley 
certainly have a central pith, though traversed by ducts, but there is no 
development of thick-walled tissue to correspond to that found in both 
Rhizophora and Zea. On the other hand, farmers hold that if a horse- 
harrow is used among cereals after the 4 tillering 5 or ‘ unbranched * roots are 
well established, the plants do not stand so well and are apt to lodge badly. 
If the ‘unbranched ’ roots are to render support to the plant, then it would 
be expected that their mechanical tissues would be developed at least as much 
as, if not more than, those of the branched roots, since the latter certainly 
do support the plant. Some idea of the mechanical strength of a root is 
given by the ratio of the area of the stele to that of the central space. 
Referring to Table I, it is seen that in the unmanured and in the nitrate 
only plants this ratio is considerably higher for the branched than for the 
‘ unbranched 5 roots, but in the superphosphate and nitrate plants the ratio 
is the same for both root types. Presumably then, in the last-named 
manurial series, the support offered by the ‘ unbranched ’ is equal to that 
offered by the branched root, and this may be significant in view of the 
fact that superphosphate manuring checks the lodging tendency which 
occurs in plants receiving only nitrogenous fertilizers . 1 It is therefore 
probable that the ‘ unbranched 5 roots do serve to some extent as ‘ buttress 9 
or supporting roots. 
Most probably the main use of the ‘ unbranched ’ roots is to be found 
in connexion with the plant’s nutritive supply. The roots appear when 
the plant is beginning to grow vigorously, and consequently when it is 
needing a good supply of water and food which must be obtained chiefly 
from the soil. The ‘ unbranched ’ roots are so constructed that they are 
specially adapted to meet such a demand. They are invested with root- 
hairs throughout their entire length, so that their absorptive area is 
considerably larger than it is in the branched roots, where absorption can 
only take place in the parts behind the growing root-tips, where the root- 
hairs are still functioning. The number of large xylem vessels is increased, 
so that there are more passage-ways along which the water can travel with 
ease. The large ducts in the pith are also probably used for the trans- 
location of water, while the unthickened condition of practically all the stelar 
tissues allows water to pass from cell to cell with considerable rapidity. 
It may be suggested that the development of lateral branches in the 
1 Purvis, O. N. : The Effect of Potassium Salts on the Anatomy of Dactylis glomerata. Journ. 
Agric. Sci., vol. ix, Part IV, Oct. 1919, p. 339. 
