Hunter.—The Aerating System of Vicia Faha. 63 j 
exchange of gases which is necessary for root-respiration, the gases must; 
either travel long distances from the aerial breathing organs, or must enter 
and leave the root by diffusing through the outer walls of the limiting layer 
of cells. Whatever system is employed must provide for an efficient 
exchange of gases—the removal of carbon-dioxide, and the provision of 
oxygen. The following statement by Jost 1 is of interest in this connexion 
‘ An investigation of the intercellular space system of all plants, whether 
aerial, subterranean, or submerged, teaches us that the accumulation of 
carbon-dioxide and deficiency in oxygen never reach a degree worth con¬ 
sidering, and hence the means at the disposal of a plant are always 
sufficient for maintaining a gaseous exchange. Carbon-dioxide to the 
extefnt of 5 per cent., and oxygen as low as 8 per cent., are seldom met 
with in intercellular spaces, and Pfeffer and Celakowski have shown that in 
the interior of living cells oxygen is never wanting.’ Livingston 2 regards 
the root hair as having the twofold function of absorbing soil water, 
and acting as a breathing organ. 
Wacker 3 is of the opinion that 
land plants are unable to supply 
oxygen to their roots by way of 
their aerial breathing organs. 
Norris 4 has shown that air pas¬ 
sages can be produced in the 
cortex of roots of Zea mats, and 
that the development of these 
passages seems to depend upon Fig. 4. Cortical cavities in old internodes, x ipo. 
the quantity of air available in the 
medium surrounding the root. Poor aeration resulted in the production of 
large air passages, but the roots of plants in media where there was a suf¬ 
ficient supply of air had normal intercellular spaces. Investigations by the 
present writer 5 & suggested that artificial soil aeration resulted in increased 
growth of plants. This has been confirmed by more recent work, and 
similar results have been obtained in water-culture experiments which have 
been carried out at the Rothamstead Experimental Station. 0 From this 
evidence it would appear that the root does obtain some of the oxygen 
it requires by means of the root-hairs, and that, at any rate in some cases, 
this must be considerably augmented by a supply from the stem tissues 
to the living cells of the older portions of the root. To apply this to the 
1 Jost: Plant Physiology, p. 195. 
2 Livingston : ibid., p. 116. 
- • 3 Wacker : Die Beeinflnssung des Wacbsthums der Wurzeln durch das umgebende Medium. 
Jahrb. wiss. Bot., 32, pp. 71-116. 
* Norris : Proceedings Bristol Naturalists’ Society. Fourth Series, vol. iii, pp. 134-^. 
& Hunter : Proc. Univ. Durham Phil. Soe., vol. iv, Part 4, p. 186. 
; 6 Hall, Brenchley, Underwood : Phil. Trans. Royal Soc., B., vol. 209, p. 194. 
T t 
