THE ACTION OF ONE CROP ON ANOTHER. 
375 
appreciably the baleful effect of the grass. Subsequently the separa- 
tion was made quite complete by growing the grass in separate iron 
trays, which fitted like a collar round the trees on the surface of the 
soil : the trays were perforated at the bottom, a sheet of fine copper 
gauze being placed over the perforations to prevent any downward 
passage of the grass roots. It was impossible with such an arrangement 
that any effect which the grass might have on the trees could be ex- 
plained by its sucking up the water and nutriment from the soil in which 
the trees were growing, but must have been due to what passed down- 
wards from the grass to the trees ; yet the deleterious effect of the 
grass persisted with scarcely 'any abatement. The results of various 
series of such experiments may be summarized as follows, the growth 
of the trees without grass, measured in various ways, being taken as 
the standard of comparison, and represented by 100 : 
No Grass. 
Grass. 
No Gauze or 
Trays. 
Gauze. 
Trays and 
Gauze. 
Vigour . 
IOO 
59 
61 
66 
The effect of the grass, it will be seen, diminishes somewhat as the 
separation of it from the tree-roots becomes more complete, but only 
to such a small extent that it may well be due, not to the more perfect 
separation at all, but to the reduced vigour of the grass itself, conse- 
quent on its growth having been restricted by limiting the extension 
of its roots ; for in numerous other experiments, which cannot be 
quoted here, it was shown, as might have been anticipated, that the 
baleful effect of surface growth is more or less proportional to the vigour 
of that growth. 
By using a collar with a larger central opening, the arrangement 
just described was adapted to growing plants other than trees in pots, 
with and without surface growth ; and in these cases the trays were 
made of earthenware, their construction being evident from a glance 
at fig. 56. The perforations in the trays were covered, as in the former 
case, with a layer of copper gauze, and in all the check experiments 
there were similar trays with gauze, containing the same weight of 
earth as those wherein a surface crop was grown. The pots measured 
16 inches in diameter internally, and contained about 70 lb. of soil, 
the trays containing 20 lb. ; all the water supplied was added to the 
trays, and allowed to soak down through them into the pot below, 
the pots being kept up to the standard weight by frequent weighings. 
In this way it was ascertained that the deleterious action of grass on 
trees was only a special instance of a general action : sixteen kinds of 
plants have been examined as regards their sensibility to the effect of 
surface growth, and all have been found affected ; whilst the various 
plants grown as a surface crop number six (not counting different 
varieties of the same species), and all have been found to behave in a 
