Sept. 30, 1886 | 
although an accurate description of each of the two 
geraniums or of the two oaks might require very different 
wording. 
It has also become more and more evident that 
although we cannot ascribe all variations to their causes 
—very often, indeed, we cannot even suggest causes for 
them—there are nevertheless numerous deviations from 
the normal, so to speak, exhibited by plants which can 
be distinctly referred to certain deviations from the 
normal on the part of their environmeat. 
To illustrate this we may take the case of two plants of 
that very common weed, the Shepherd’s Purse, growing at 
different ends of the same small plot of ground: the soil 
is sandy, and so much alike all over as to be regarded as 
the same everywhere, nevertheless the plant at one end 
is large, more than a foot high, and luxuriant, with many 
leaves and flowers, and eventually produces numerous 
seeds, whereas that at the other end is small, less than 
4 inches high, and bears but a few stunted leaves and 
three or four poor flowers and fruits. The cause of the 
difference is found to be the different supply of water in 
the two cases, and if any one doubts that this may be so, 
let him try the experiment of growing two or more speci- 
mens of this weed in pots: the pots to be new, filled with 
soil which has been thoroughly mixed, and all the pots ex- 
posed to the same conditions—z.e. practically the same— 
except that those of one series are watered sufficiently 
often, and those of the other only just sufficient to keep 
the plant actually living. The experiment is easy and 
conclusive with such a weed as the above. Now, it is 
just such experiments as that above described—some of 
them equally simple, others less so—that the physiologist 
devotes much of his attention to, and in just such a 
manner has been gathered together a nucleus of informa- 
tion around which more knowledge can be grouped. 
I may make these points clearer by again quoting an 
illustration, and, not to confuse or mislead you by going 
too far afield, I will keep to the same line of investigation, 
partly because it is quite as simple and conclusive as any 
other of many that might be selected, and partly because 
it may be possible to set before you some facts which are 
interesting or even new to you. 
It has been found that in some cases where two plants 
are growing in the same soil and under the same con- 
ditions as above, but where one plant receives less water 
than the other, that the dwarfed drier plant is more 
hairy than the larger and luxuriantly growing plant, which 
has been well watered. On looking more closely into 
this matter it turns out that the extra hairiness is (in 
some cases at any rate) simply due to the fact that the 
hairs are closer together, because the little cells on the 
outer parts of the plant which grow out into hairs do not 
increase so much in length and superficial extent as those 
on the well-watered plant, and thus the hairs stand 
thicker together on the same superficial area of the organ 
—of a leaf, for instance. In other cases, however, the 
hairs are really increased in numbers and length—the 
plant is absolutely more hairy. It will be noticed that 
details concerning growth and turgidity, and of the in- | 
fluence of various minerals, and so on, are not under 
consideration here. I am not asserting that all cases of 
hairiness in plants are to be ascribed to this cause ; but 
it does occur, as stated, and the point is a curious one 
in view-of the fact that very many plants which grow in 
sandy dry soils are conspicuously hairy, whereas allied 
species growing in or near water, or even only in moister 
Situations, are devoid of conspicuous hairs, or even quite 
smooth. 
The above peculiarity is not confined to leaves and 
stems, moreover, for experiments with roots have shown 
that the root-hairs, which are so important in collectiag 
moisture, &c., from the soil, can be made to appear in 
enormous numbers when the root is kept in a soil which 
is very open and only slightly moist, whereas none or 
NATURE 
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373 
very few are developed on the same roots growing in 
water : this again is in accordance generally with the fact 
that the roots of land-plants growing in light soils develop 
innumerable root-hairs, whereas those of water-plants do 
not thus increase their surface and points of attachment. 
I cannot here go into all the interesting facts known about 
these hairs, but it will be sufficient if you bear in mind 
the main points just mentioned. 
Let us now vary the experiments a little. It is obvious 
that we might suppose any number of differences in 
the amount of water given to the plants used in the 
experiments described above ; but it would be found, as 
matter of fact, that however little be the quantity of water 
given to the soil in which the dwarfed plant is, compared 
with that put into the soil in which the luxuriant plant grows, 
the actual weight of water will nevertheless have to be con- 
siderable, taking the whole life of the plant into considera- 
tion—there will be more used than you probably know, 
moreover, because the soil itself will no doubt condense 
and absorb some from the atmosphere during the night. 
There is a minimum of water absolutely necessary, and if 
the plant does not obtain this it will die. Its death will 
be ushered in by drooping and withering of the leaves, 
stem, and roots, and this condition, in which the functions 
of the plant are interfered with beyond a certain point, 
passes into a condition of disease. 
Now take another case. We might so arrange the 
experiment that we poured and continued to pour too 
much water into the soil. Here again it would be found 
that a condition of disease eventually sets in—z.e. a con- 
dition in which the functions of the plant are again inter- 
fered with beyond a certain point. The symptoms and 
progress of the disease will be very different in the latter 
case, however, from those in the former. It may also be 
mentioned that in neither experiment is death inevitable 
if the disturbing cause is removed soon enough—ze. if 
sufficient water be added in the first case before the 
cells have ceased to be able to take it up, or if the 
previous conditions of the soil are restored soon enough 
in the case of the over-watered plant. 
Here we come to a matter which is less simple than 
may appear at first sight. You will note that the problem 
in the latter case is to restore the previous conditions of 
the roots and soil soon enough; I put it thus, because 
the conditions of the roots and soil may soon be very 
profoundly altered by the over-watering. 
To understand this, it is necessary to become a little 
more fully acquainted with the condition of affairs in what 
may be called the normal case, where the soil is light and 
open, and plenty of water but not too much is at the dis- 
posal of the roots. Such a soil will consist of innumerable 
fine particles, of different shapes, sizes, and composition. 
No doubt there will be grains of quartz, particles of 
broken up vegetable matter, and little rugged bits of 
stones containing various minerals; each of these tiny 
fragments will be covered with a thin layer of water, and 
you would probably be greatly surprised if I were to go into 
the proofs showing how extremely tenacious of its water- 
blanket each particle is. It may be enough for our present 
purpose if you accept the fact that it requires enormous 
force to deprive the particles of the last traces of their 
water-layers ; they will give off some—or in some cases 
even a good deal—rather easily, and in fact when the 
layers become of a certain thickness no more water can 
attach itself to the particles, but it falls away and the soil 
remains saturated, as we say. 
Now these particles of soil, each enveloped in its water- 
blanket, are not in close contact ; there are spaces between 
them, and these interspaces influence the quantity of 
water which can be held back by the soil. 
Let us suppose such a soil perfectly dry ; the particles 
above referred to being irregular in shape and size, and 
only roughly in contact at various points, the interspaces 
If water be then added in some 
