554 Brenchley and Jackson . — Root Development of Barley and 
many were broken off and overlooked, but that this was not the case was 
proved by the other experiment, previously described, in which blocks of 
soil containing the whole root system were removed and soaked and washed 
out under conditions allowing of the recovery of all broken pieces. Even 
under these circumstances the longest roots were only 9 in. long, and the 
results compared closely with those obtained in the field. 
The results of these experiments may be considered from two 
aspects : 
1. Influence of environmental conditions, other than manuring, upon 
root growth. 
2. Influence of different types of manuring on root growth. 
1 . Influence of environmental conditions , other than manuring , upon root 
growth. The general type of the root system seems to be a more or less 
constant character of the species concerned, but wide variations within the 
type may occur through the influence of soil factors. 
The environmental conditions presented by the field and pot cultures 
were so radically different that it is not surprising to find the root systems 
of both wheat and barley developed differently in each case. The lack of 
penetration of the barley roots into the hard field subsoil is a well-known 
phenomenon observable in many plants. Markle 1 indicates that with 
desert plants a layer of soil that is difficult of penetration may cause much 
distortion of roots entering it. Also, roots will often turn abruptly from 
a layer of clay and follow a thin layer of sand or fine gravel containing much 
less water but more easily penetrable. This was paralleled by the behaviour 
of barley roots in utilizing worm-tracks or other passages when opportunity 
offered. 
Shepperd found with maize that the roots penetrated less deeply when 
a layer of shale was present under the surface soil, rendering the lower layers 
dry and uninviting to plant roots. 
Associated with the question of penetrability is that of the aeration 
of the soil and its effect on root growth. In experiments in India Mr. and 
Mrs. Howard 2 found that for adequate development the roots must have 
abundant room in the soil for rapid growth and the space must be 
adequately ventilated. The rooting habit was greatly influenced by the 
soil conditions. Linseed, when grown on alluvium, was shallow rooted, but 
on deeper soils penetrated much farther. This last effect was attributed 
to the cracking of the upper soil, whereby the subsoil was more efficiently 
aerated and deeper rooting rendered possible. Similar results were obtained 
by Cannon, 3 who states that deep-rooted plants are less dependent on good 
1 Markle, M. S. (1917) : Root Systems of Certain Desert Plants. Bot. Gaz., lxiv, pp. 177-205. 
2 Howard, A. and G. L. C. (1917) : The Economic Significance of the Root Development of 
Agricultural Crops. Agr. Jour., India, Special Indian Sci. Congress Number, pp. 17-28. 
3 Cannon, W. A. (1915) : On the Relation of Root Growth and Development to the Temperature 
and Aeration of the Soil. Amer. Journ. Bot., ii, No. 5, pp. 211-24. 
