106 GARDENING AS A SCIENCE. 
as a silky, white powder, which, under a lens, exhibits the appearance of rounded, 
transparent, colourless granules, the magnitude of which varies very much with 
the plant from which they were obtained." 
Microscopic observers are at variance on many points, but all seem to agree, 
however, that each granule is enclosed in a membranous covering. 
When starch is put into cold water, and the tem.perature gradually raised to 
the boiling point, this membrane gives way, as if by the expansion of the matter 
within ; solution takes place, and a translucent jelly, familiar in the common 
arrow-root, is produced." Vol. lY. part II. 505. 
If starch be boiled slowly, and for a considerable time, in a little diluted sulphuric 
acid, it is converted into sugar, of the variety termed Sugar of Grapes; the 
sulphuric acid suffering no change whatsoever. 
2. Woodi/ tissue consists of longitudinal tubes or fibres, the rudiments of the 
matter of the future wood. 
3. Vascular tissue, of which the spiral vessels or tubes are the 
type. They are found in the medullary sheath, in the veins of the 
leaves, but rarely in the wood or bark. If a young, green twig of 
elder be cautiously broken, these spirals will be drawn out ; and, 
infinitely fine and delicate as they are, they are sufficiently strong 
and elastic to sustain a piece of the twig half an inch long, and thicker 
than a crow-quill. This true, original spiral vessel, contained within 
masses of cellular tissue ( Parenchyma ) is represented at § of the 
annexed figure. 
" Diicts^' is a term recently introduced for those " transparent tubes, the sides 
of which are marked with rings, bars, or transverse streaks. They are slight 
modifications of the spiral vessel, differing principally in being incapable of 
unrolling ; and, in some cases, in the turns of the spiral fibre being distant or 
broken, or even in appearance branched." — {Elem. of Botany.) 
Ducts, it should appear, comprise all those vessels which, some years ago, were 
classed among the modifications of the spirals, and termed 
punctuated^ annular^ and reticulated vessels. In a wood- 
cut of a publication on vegetable physiology, by the Society 
for the Diffusion of Useful Knowledge — never completed — 
they were thus figured ; — S is the spiral or supposed type ; 
ffl, the annular vessel, or most simple modification; c 
portions of the cellular tissue ; d d^ a punctuated or dotted 
vessel, with irregular, distant spiral threads. The reticulated, 
or netted vessel, was stated to be found in few plants, one of 
which is the balsam at its full growth, and chiefly in its roots. 
These vessels were considered as conduits of the ascending sap ; but though 
all of them may, at times, contain fluids, it is more than probable that the opinion 
of Mr. Knight, before mentioned, is correct ; this, however, is not the place 
