38 Jackson. — Anatomical Structure of the Roots of Barley. 
‘ unbranched 5 roots is influenced by light, since these roots occasionally 
come up to the soil surface and are consequently nearer the light than are 
the branched roots. That this is not the case is clear from the fact that 
ultimately lateral branches are developed ; moreover, Newton 1 has shown 
that for terrestrial plants grown in water, branching takes place just as 
freely in the light as in the dark. The dormancy of the lateral branches 
is explained by the fact that the ‘ unbranched ’ roots develop when the 
plant is making vigorous growth and needs a plentiful supply of water with 
its dissolved nitrogenous and mineral constituents; if the roots were to 
produce laterals immediately, then thickening of the stelar tissue would 
follow and the translocation of water would become slower. On the other 
hand, when the laterals are dormant, the tissues of the stele remain 
comparatively thin-walled and water can very readily pass through the 
roots to the stem and leaves, and in addition the root-hairs are able to 
continue functioning along the whole length of the root. 
The theory that the ‘ unbranched ’ roots are chiefly connected with 
the food- and water-supply of the plant receives further support from the 
fact that these roots are only formed during the early stages of the plant’s 
vigorous growth. Researches on the development of root and shoot 2 
showed that the formation of ‘ unbranched ’ roots had entirely ceased by 
the time the plant had finished its vegetative growth and was entering on 
its reproductive phase. At this period of the plant’s history the nitrogen 
and ash constituents are migrating steadily from the straw into the grain, 3 
so that there is no need for a large root-absorbing area. On the other 
hand, if the ‘ unbranched ’ roots functioned chiefly as buttress-roots, the 
plant would need them even more when the heavy grain is being formed, 
but that is just the time when their development ceases. Therefore the 
most probable function for the ‘ unbranched ’ roots is to ensure a good 
supply of water, &c., when the plant is in a condition of strong vegetative 
growth. 
Summary. 
1. The root system of a well-developed barley plant consists of two 
types of roots : (a) a thin branched type, and ( b ) a thick ‘ unbranched ’ type, 
with very abundant root-hairs. 
2. A branched root possesses a much-thickened stele with a single large 
axile vessel and six to eight xylem groups all bounded by a very thick-walled 
endodermis. In an ‘ unbranched ’ root neither the endodermis nor the stelar 
tissues are thickened, the xylem groups number from twelve to sixteen, 
1 Newton, L. M. : Conditions which affect the Branching of Roots. Rept. Mich. Acad. Sci., 
Lansing, 1911, xiii, p. 200. 
2 Brenchley and Jackson : loc. cit. 
3 Brenchley, W. E. : The Development of the Grain of Barley. Ann. Bot., vol. xxvi, No. ciii, 
July, 1912, pp. 913-19. 
