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VEGETABLE PHYSIOLOGY, 
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DICOTYLEDONOUS STEM, THREE YEARS OLD. 
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d e 
A DICOTYLEDONOUS STEM FOUR YEARS OLD. 
a, pith and wood of the first year; i, c, and d, layers 
of wood of the second, third, and fourth years ; e, the 
four thin layers of fiber. 
surface of that formed in the preceding year, and as there is a little difference in the size or amount of 
internal deposit in the last cells of one year and the first of the next, we find a line of demarcation 
between each period of growth, these being the lines commonly known by the name of “ annual rings.” 
At the same time the liber bundles of the bark, which 
have been pushed outwards to make room for the new 
wood, receive a new layer on them inside; a layer which 
is very thin, often forming a mere plate, thinner than 
paper, but as regular in its production as that of the 
wood; sometimes these may be very distinctly traced, by 
their coming apart very readily when dry, as in the lace- 
bark trees, or in the Lime, the liber of which forms the 
material called bass, used for matting, &c. 
Each succeeding year witnesses a similar change, and 
thus the stem here acquires, simultaneously with the 
new lengths at its growing points, a new layer of thick¬ 
ening all over the older parts, a process which does not 
occur in Monocotyledons, or, in a few where it appears 
to do so, in a totally different way. "We have said in 
each succeeding year, and used the term “ annual rings,” 
because in our temperate climate these words represent 
the truth in almost every instance; the alternations of 
the seasons are so regular that the periods of growth are 
annual. But it is not always so in other climates, perhaps 
not so often as has been imagined; at all events, it is 
known that the trunks of arborescent Cactaceee present 
rings indicating periods of growth extending over several years ; and it is the same in the Cycadacese. 
On the other hand, it has been stated that a temporary interruption of growth, such as that produced 
by the loss of the greater part of the foliage early in the year, will cause two rings for that year ; 
and rings have been counted in such numbers in 
tropical trees, that we hesitate to regard them as 
representing periods of existence so long as a year, 
and incline to believe that less marked changes 
than our alternations of winter and summer, may 
suffice in those regions to produce the alternate 
arrest and re-awaking of development. 
The roots of Dicotyledons are naturally, that 
is in plants raised from seeds and not from cut¬ 
tings, direct continuations of the lower end of 
the stem. Their solid structure, therefore, is 
formed of prolongations downwards of the fibro- 
vascular bundles, but these soon coalesce, and the 
pith is lost; there are no medullary rays, and 
the bark is a rind devoid of liber bundles and 
more soft and succulent than that of the stem. 
The wood of the root is increased by successive 
layers all over the old parts, simultaneously with 
the formation of the new layers in the stem; 
but the structures are much less easy to make 
out here, from their solid condition and the 
complication arising from the tortuosities and 
close ramifications of the roots. The main chan¬ 
nel of absorption is through the spongy rind of the root and its younger layers of wood, up which 
the sap rises into the bark, cambium layer, and young wood of the stem. The old wood of the root, 
like that of the stem, becomes, after a time, consolidated by internal deposits into a dense structure ; 
in some trees almost into a solid mass. This older solid part of the trunk is called the duramen, or 
heart-wood; the outer and still active layers, the alburnum, or sap-wood ; these regions often present¬ 
ing even a difference of colour. 
SECTION IN THE SPRING OF THE FIFTH YEAR. 
The white space indicates the swelling cambium layer. 
STEM AT THE END OF THE FIFTH YEAR. 
The layers of fiber are too thin to be represented in the drawing. 
