—— 
Sept. 30, 1886] 
NATURE 
527 
few or no root hairs formed on their surfaces ; whereas it 
may be readily shown that the roots of the same plant 
growing in a well-aérated open soil, which is scarcely 
moist to all appearance, will be densely covered with a 
close-set pile of hairs. Indeed it is by means of the 
millions of root-hairs on its rootlets that a sunflower or a 
bean, for instance, obtains the enormous quantities of 
water necessary for its needs from soil which, to our 
rough perception, seems to be dry. 
I cannot here go into all the proofs that such a soil is 
by no means so dry as it looks; but will simply remind 
you of what was said above as to the enormous force 
with which the minute particles of rock, &c., which form 
“soil” retain their hold on the thin films of water which 
constitute what have been termed their water-blankets. 
This is certain, that a healthy well-rooted plant can take 
up water from a soil which is to all appearance air- 
dry ; whereas a plant which has not yet had time to 
develop its root-hairs in sufficient numbers to take these 
firmly adherent water-films, from numerous particles of 
soil, would droop and wither. 
Of course it must be borne in mind that we are speaking 
ofland-plants such as we commonly meet with on ordinary 
dry land: in the case of plants which flourish in bogs or 
in water there are corresponding differences in the struc- 
tures of their roots agreeing with the differences of en- 
vironment. Even such plants need air at their roots, 
and an excellent illustration of this is is afforded by some 
willows. Our common osier and other willows grow, as 
you are aware, in low-lying damp and even boggy places, 
often flooded: now, it has been found that, if young 
willows are planted too deep in the soil, they very soon 
send out new roots—adventitious roots they are often 
called—close to the surface of the soil, and these roots 
soon do all the work. There is no doubt that this power 
enables these willows to live in places that would be fatal 
to them otherwise; and the same is true of some other 
plants. 
Enough has now been said to show you how necessary 
it is that some care should be exercised in watering plants, 
or in exposing them to conditions different from those to 
which they are accustomed; and, it need scarcely be 
added, apparently mysterious diseases may sometimes be 
explained when it is shown that such precautions have 
been neglected. Any one can quote instances of plants 
which will grow in some soils and not in others, but no 
very satisfactory reason is afforded by simply saying that 
the one soil is suitable and the other not: however, all I 
have attempted to show you is that some soils are not 
suitable for some plant; because the plants in question 
need more air at the roots than these particular soils can 
afford them under the circumstances. , 
Many plants flourish in an open soil with plenty of 
sand in it, but will not grow in a stiff wet soil. This is not 
necessarily because the heavier soil does not contain the 
right food-materials, but because its particles are so 
small, so closely packed, and so retentive of moisture, 
that the root-hairs do not obtain sufficient oxygen : more- 
over, the very damp state of the soil does not favour the 
development of the numerous root-hairs necessary, as we 
have seen. 
at length into this point,—root-hairs and roots cannot 
grow or act unless the temperature is favourable, and we 
have plenty of evidence to show that a close wet soil may 
be too cold for the roots at a time when an open drier 
soil (exposed to similar conditions as regards sunshine, 
&c.) would be of a temperature favourable to their 
growth. Many a pot-plant receives an extra over-dose 
of water because it is drooping from the roots being too 
cold to act properly. The opening up of stiffer soils by 
means of the spade or plough, or by the addition of other 
kinds of soil, such as sand, burnt lime, &c., or by means 
of drainage of various kinds, is thus to be regarded as a 
_ means of letting in air and therefore oxygen to the roots. | about a “diseased” condition of the roots. 
Nor is this all,—though I cannot here enter | 
“ Sweetening the soil” is an expression one hears used 
by planters and others: this is often no doubt their way of 
expressing the fact that the air thus let in does so much 
to turn the noxious substances which have accumulated 
into other substances which the root-hairs of the plant 
can take up with profit. The exposure of certain soils to 
sharp winter frosts in part benefits the plants subse- 
quently grown in it, because air can make its way into 
the cracks produced as the particles crumble: there are 
other advantages also due to the “weathering” of soils, 
of course, as also to the addition of lime, &c., but I am 
purposely abstaining from referring to points concerning 
the nutrition of plants as generally understood. 
Let me shortly call your attention to a few other 
practical applications of the knowledge briefly summed up 
above. It is well known that a good deal of experience 
has been brought to bear on the question of what trees 
are the best to plant in or near large towns: there are 
very many facts to be considered. It is not sufficient to 
find a tree which will accommodate itself to the possi- 
bilities of the annual rainfall, or a diminished supply of 
sunlight throughout the year, and so on; nor is the 
problem solved when a tree is found that will put up with 
traces of acid gases in the atmosphere, and, as may 
follow, the accumulation of acids in the soil, and conse- 
quent alterations in its chemical composition. In many 
cases trees have been found to die as they grew older 
because the pavement or asphalt over their spreading 
root-system prevented proper aération and a_ proper 
supply of aérated water to their root-hairs : imagine the 
effect of a few days’ hot summer sunshine on roots just 
beneath the pavement of an exposed street! It is true 
the cover may prevent rapid evaporation, but it also 
shelters the soil from the well-aérated rain-drops ; more- 
over, such sheltered roots will at certain seasons grow up 
to the surface of the soil and in contact with the lower 
surface of the pavement. Then there is the question of 
drainage. If the water which does find its way in slowly 
accumulates and becomes stagnant, the results are as dis- 
astrous or even more so; yet it is obviously a difficult 
matter so to arrange things that the accumulated surplus 
water of certain seasons shall pass away below, acting 
like a suction-pump and drawing in air after it, and still 
fulfil the other requirements hinted at above. I leave 
out the question of exhaustion of the soil—the dead leaves, 
&c., being carefully removed. Can we wonder that there 
are so few trees to choose from that will stand such treat- 
ment? The fact that there are some only accords with 
what has been already stated—that plants vary in their 
requirements and powers ; and no one doubts that the 
variations have been influenced by variations in the 
environment. 
We have now seen to a certain extent how variations 
of a particular kind may affect a plant. The plant 
responds to a certain extent—it is, as some people say, 
“plastic” —but if the limits are reached and slightly 
overstepped, the variations on the part of the plant 
become dangerous to its existence, and the plant becomes 
diseased and may die. 
Not to dwell upon hypothetical matters, I will content 
myself with saying, in conclusion, suppose a variety of 
a given plant grows in damp places and has roots which 
form few or no root-hairs, and suppose an individual of 
that plant to become transferred to a more open soil: I 
have shown you reasons for regarding it as probable that 
the latter individual might produce more root-hairs and 
thus adapt itself to the altered conditions. If such a case 
happened, it is by no means improbable, but the contrary, 
that other circumstances co-operating or adverse would 
decide certain problems of importance to the existence 
of that particular individual. 
But the main object of this lecture has been to show 
you how very complex the conditions may be baer bring 
t is no 
