Wey 
cells, with, occasionally, a central vertical cord of fibre cells, as, for instance, in 
the mosses. Such a stem, however, would be unsuited to plants in which great 
strength is required, and We, accordingly, find that, in all plants above the mosses, 
the stem is made up partly of parenchymatous cells and partly of woody tissue 
and vessels of different kinds, by which the requisite toughness and strength are 
produced. In these stems, then, we notice two systems—the parenchymatous, or 
common cellular system, and the fibro vascular. Vegetable cells are divided into 
two groups. When they are of such form that, combined together, their ends merely 
come in contact without perceptibly overlapping, they are called parenchyma- 
tous ; but when elongated and pointed at their ends, so that, in combination, they 
overlap each other, they are termed prosenchymatous. But these extreme forms 
are connected by all sorts of transitional ones. Formerly, all elongated organs 
found in plants were supposed to have an entirely different origin from the cells, 
and were described under the names of woody fibres and vessels or ducts. It 
is now, however, known that they are all derived originally from ordinary cells. 
The parenchymatous system grows in any direction, according to circumstances, 
either longitudinally, by which the stem is lengthened, or horizontally, by 
which it is increased in diameter, The fibro-vascular system only grows longi- 
tudinally, and thus forms cords and bundles, which are distributed vertically in 
the midst of the parenchymatous to strengthen the stem. The parenchymatous 
is, theretore, termed the horizontal system of the stem, and the fibro-vascular the 
longitudinal, or vertical system. 
In the embryo state, the stem of an exogen is entirely composed of paren- 
chyma ; that is, a tissue composed of simple cells with thin walls, whose length 
does not exceed their breadth, or in which the proportion of the two diameters 
does not vary to any great extent. As soon as growth commences, some of the 
cells become developed into spiral vessels and wood cells, forming, at the end of 
the first year, a zone, consisting of two parts, the first composed chiefly of spirab 
vessels, and known as the medullary sheath, and outside this, and closely con- 
nected with it, another part, composed of wood cells; this zone is formed round 
the central mass of parenchyma, which, from this time, we will call the pith ; it 
is not, however, a perfect circle, for itis interrupted at certain points by projec- 
tions of the pith in radiating lines, which pass through it and connect with an, 
external layer of parenchymatous tissue, which is called the bark. We have, 
then, as the result »f one year’s growth :—1l. A central mass of parenchyma, 
which is called the medulla, or pith; 2. An interrupted zone of wood cells, and 
vessels forming the wood; 3. The radiating lines connecting the pith with the 
bark, called the medullary rays; and 4, An external zone of parenchymatous 
cells or the bark. Such is the structure of all exogenous stems which die 
yearly. 
But our stick does not belong to an annual exogen ; the stems, however, of 
plants which live more than one year, at first resemble those that die yearly, 
except that the wood formed is generally firmer and in larger proportion. Dur- 
ng the second year, another zone of wood is formed on the outside of the one of 
