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VEGETABLE PHYSIOLOGY. 
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Kg. 3. 
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Slice of hexagonal cellular 
tissue magnified. 
Fig. 4. 
posing the chief part of the thickness, is formed of cells either round, oval, or of irregular shapes, (Fig. 
1. d, e,f,) only toucliing at certain points, and thus leaving ititercellidar passages heUveen them. Such 
is the condition of cellular tissue in all the more soft and succulent parts of plants. 
In other regions where somewhat greater strength is necessary, we find the cells of other forms, 
„j^ ,, either square or oblong, so that they can be built up 
together like the stones or bricks of a wall, without 
leaving interspaces ; or, as is often the case, of a twelve- 
sided figui'c, like the cells of a 
honeycomb, wliich also admits 
of their being packed together 
Magnified portions of cemdar tissue. without interspaces, and is the 
form which globular bodies assume when squeezed together in a confined 
space. The square or oblong form is very common in the firmer cellular 
textm-es, such as cork and other structm-es forming the rind or bark, and 
intended to protect more delicate textiu'es beneath them (Fig. 2). The 
twelve -sided form is more frequently met with in internal parts which have 
enlarged by the growth or swelling of the cells after they have become 
enclosed bj' fii-mer structures, the change from the globular to the angular 
form resulting from the mutual pressure (Fig. 3). 
A third form of the cell is produced when the young- cell grows in such 
a manner that it becomes elongated into a long tube closed at both ends, and where this growth takes 
place so that the tubes taper to a point at each end, they become interlaced with each other, and form 
a very much tougher texture than where the cells come in contact by flat sides and ends (see 
Fig. 4, where the cells are magnified). This form is met with in almost all woody structures, 
and in all then- fibrous or stringy textures, as in the bass or liber of the bark, in the ribs of the 
leaves, &c. 
The foregoing forms are all composed of delicate membrane in the fii-st instance, and 
the cells of globular or irregular form do not usually alter much subsequently ; but the square 
and oblong forms, which serve for the construction of the firmer cellular textures, and the 
elongated tapering cells of the woody tissues, begin very early to receive new laj'ers all over 
their internal surface to thicken and strengthen them. These layers of thickening may be 
compared with coats of paint applied in the inside of any hollow body ; they are deposited 
from the fluids contained inside the cells, and in proportion as they are successively pro- 
duced, the walls of the cells, as they are called, grow thicker, while the ca\'ity is gradually 
encroached upon and diminished. This deposition of new 
layers goes on to such an extent in some cases, that the 
cavity becomes almost filled up, and we see this very re- 
markably in sections of the heart-wood of the more solid 
kinds of trees, but especially in the sohd woody substance of the 
stones or shells of many fruits (Fig. 5, a, 5,). But it is evident that 
as the walls are tliickened they become less readily permeable by the 
fluids bringing new materials for the growth ; accordingly we find a 
beautiful provision to obviate this difficulty. When the first layer 
of thickening is applied to the original membrane, a number of 
little points are left uncovered, and the successive layers as they 
are deposited one upon the other, always leave these points clear ; 
thus when the walls have acquired even very great thicloiess, 
passages exist leading out at these uncovered points to the original 
membrane, so that fluids can readily penetrate into the interior, and 
fllter through from one cell to another. 
This is the condition of the elementary organs in most of the 
firm and solid parts of plants ; but there are many circumstances 
in which a greater lightness or more elasticity is requisite, in those 
structures which are to give the necessary strength. This is efiected by a peculiar mode of develop- 
ment of the thickening substance applied upon the internal sui'face of the cells. Instead of extending 
all over the surface, it may be deposited upon certain lines extending round the walls, and the most 
frequent form is that of a spiral line running round and round the wall from one end of the cell to the 
other. The thickening matter laid down upon this line then presents itself as a spiral ridge project- 
Magnified slices from the stones of the 
Plum and Pear, showing the greatly 
thickened walls of the cells; the 
shaded portions are the cavities. 
