^98 
Supplement to the "Tropical Agriculturist." [Ocr. 1, 1896. 
up, (lie possibility suggested itself that a key 
to the solution of the problem might be fouiul 
in nn nppenl to the plant itself, namely, in a 
furflier study of the means by wliicli it collected 
its mineral food, That a plant takes up some 
of its food (most of its nitrogen, for example) 
in a state of solution, or by a mere process of 
diffusion, is very probable ; but from what we 
know of the solubility ot most mineral consti- 
tuents, the mere solvent action of water, even 
if saturated with carbonic neid — that is to say, 
of the rery limited quantity of water that exists 
in even a wet soil — is wholly insuilicicnt to account 
for the solution of the mineral plant food taken 
up by a crop. 
It had, indeed, long been recognized that plants 
help themselves to at least a part of their mineral 
food by means of the solvent action of their root 
sap, the acidity of which had been noticed so 
far back as 1733 by Philip Sliller Gardener to 
the Society of Apothecaries, who wrote that 
"when the juice enters the root it is earthy, 
watery, poor, and acid" while he also drew 
attention to "that tnrt liquor oozing from the 
root of the walnut tree when cut off in the 
month of Jlay." 
In more recent kinds the German vegetable 
physiologist, Saciis, had shewn that when polished 
marble was buried in the soil in which plants 
were growing, the action of the root-sap was 
■sufficiently corrosive to etch on the surface of 
the matble the pattern corresponding with the 
paths of contact between the roots and the 
marble. 
In 1889 Sir John Lawesand Sir Henry Gilbert, 
in a paper " On the Present Position of the 
Question of the Sources <i Kitrogen of Vege- 
tation," described some experiments made with 
a -view to ascertaining how far the acidity of 
the root-sap might enable plants to dissolve and 
assimilate nitrogenous organic matter in the soil. 
In the course of that investigation the authors 
experimented on the acidity of the root-saj) of 
a large number of plants, although their quanti- 
tative determinations of acidity were not; pub- 
lished. Few, if any, other attempts appear to hare 
been made to determine the degree of acidity 
possessed by root-sap, although the fact of its 
acidity was well recognised. 
It therefore appeared desirable lo undertake 
some investigation into this acidity in the fine 
roots or rootlets of a number of plants- 
»- 
THE USES OF WOOD. 
{Continued from page 10, Vol. X.) 
tn splitting and cleaving, the case is, like shear- 
ing, aliiiost entirely one of transverse tension ; with 
this difference, however, that the force is ajiplied 
to a small area and acts on a lever (the side of 
the cleft); it acts, therefore, the more effectually 
the longer the cleft and the stiffer the wood. From 
what has preceded, it is evident that the adhesion 
of the fibres, or, better, the resistance to trausA'erse 
tension, is of great importance. Examining the 
structure, it is quite apparent that this resistance 
is greatly influenced by the shape and relative 
position of the fibres. In hard woods the cells 
do not arrauge themselves ia rows ; heiice, there 
is no natural cleavage plane? (except at th« jUtli 
rays). A knife will thus not merely have to 
separate two layers of fibre?, but has t.o cut through 
the cells tlienisulve-', while, if passing through 
coniferous wood it finds a natural pbine of contact 
of two sheets of fibres, and thus has emy work. 
Moreover, the course of the fibres in hard woods 
is rarely straight, the fibres are generally in oblique 
poBition.'s, they interlace, and if a piece of wood is 
f-plit the surface is " fuzzy " with the myriads of 
fibre.s which are not merely separated, init were 
torn in tension, the very way in wbich they off^-r 
greatest resi.staiice. For the>-e reasons liard woods 
have generally a much greater nirength in tran!^- 
verse section than conifers. Where tlii« greater 
resistance to tension is accompanied by grwiier 
flexibility, by more "give,"' as is always the ca«e 
with hard woods, tho wood becomes loupli ; a 
blow may indent but does not shatter. This tougli- 
ne^s is a combination of relatively great strength 
ill transverse longitudinal tension together with a 
fair amount of flexibility or capacity to endure 
distortion. That toughness varies widely is well- 
known. Naturally the hard woods exhibit it to a 
much greater degree than conifers. 
Hardness in wood means the resistance which 
any surface, but particularly the sides (longitudi- 
nal faces), offers to the entrance of a blunt ho<ly 
such as a hammer. The test in hardness 18 one of 
transver.-e compression of the fibres, and therefore 
depen<ls on the resistance tocoliap.'c. In a single 
fibre this resistance depends on thatof the material 
(presumat)ly about alike in all wood), on the 
shape of the fibre, and the relotive thickne»« of 
its walls. Fibres like those of hard woods with 
a hexagonal cross section and commonly scarcly 
any cell linnen or hollow, naturally beliave lika 
solid wood substance. They offer great resietonce, 
so that if the outer surface of a stick is formed 
by such fibres its hardness is very great. If, on 
the other hand, the surface layer is compose*! of 
thin-walled vessels or tracheids, like those of the 
spring wood in conifers, the wood is soft. In the 
usual test the indentation e.Ktends but a jhort 
distance ahead of the instrument (as, for instance, 
when timber is struck witii a hammer) : but if the 
test is continued long enough, the compression 
results in destruction of all the thin-walled and 
much of the thick-walled tissue of the wood, so 
that timbers, such has have been buried under 
ground, are destroyed throughout. Such a crushed 
stick continues to resist further crushing, becomes 
compacted, dense and heavy, and loses nearly 
all its bending strength &c. ; it takes up water 
rapidly, and when soaked crumbles like wood in 
the later stages of decay. Closer examination 
shows that all the thin-walled fibres have collapsed 
just like crushed pasteboard tubes, the break 
running along two or more lines the length of the 
fibre, the form of the cross section being changed 
from a hexagon to au S shape, or an approach to 
this form. 
The hardness of wood in the sense as noted is 
quite variable, even in wood of the same species, 
varying on different sides and also according to 
the portion of the annual ring exposed at the 
surface, the extent of compression and other 
circumstances. 
In nearly all wood used for construction, whether 
bridge timljer, the studding or joist of a house, or 
