74 
MORPHOLOGY OF TISSUES. 
the other. Thus a fissure arises which, in consequence of the mode of its origin, 
assumes the form of a triangular prism with concave sides (Fig. 58, z). It is 
filled with air, and becomes one of those intercellular spaces which very usually 
form in the parenchyma a continuous system of narrow channels. Not unfrequently 
the portions of the wall which bound the intercellular space grow rapidly, and thus 
it increases in size ; the cells assume irregular star-shaped outlines in transverse 
section, touching one another only at small portions of the surface, as in the 
parenchyma on the under side of the leaves of many Dicotyledons, and the 
stem of /uncus effusus. In the faces of the cell, where no other wall intersects 
them, spHttings of the homogeneous lamella may also occur locally; sometimes 
these are limited to narrowly circumscribed places, and produce flattened cavities 
in the homogeneous partition-wall. In other cases the splitting into two lamellae 
takes place in such a manner that only isolated roundish places remain 
unaffected by it; the separated lamellae continue to grow rapidly by intercalary 
growth, and bag-shaped protrusions of adjoining cells are formed which are 
separated by the fragments of the originally unsplit cell-wall (Fig. 59). In other 
cases there follows on the partial splitting of the partition-wall a local growth of 
one or both of the two lamellae (or of only one), so that a fold arises which intrudes 
into the cell-cavity, as shown in Fig. 60, _/! Finally, in some species of the genus 
Spirogyra, the septum between each pair of cells splits into two lamellae, each of 
which grows as a protrusion into the interior of the adjoining cell, and, when 
the adjacent cells separate, becomes turned inside out somewhat like the finger 
of a glove previously folded in. When the walls of cells forming a tissue split 
everywhere into two lamellae (the separation proceeding always from the inter- 
cellular spaces) and become rounded off, a complete dissolution of the tissue 
takes place into a mere mass of isolated cells. This occurs in the flesh of many 
"1 
Fig. 59.— Two rows of cells running- in a radial direc- 
tion (/, //, /// and I, 2, 3) from the cortical parenchyma of 
the root of Sagittaria sagitti/olia in transverse section ; 
a the protrusions, / the intercellular spaces between them 
(X about 350). 
Fig. 60. — From a transverse section of the leaf of 
Pinns Pinaster ; h half of a resin-passage, to the left 
parenchymatous cells containing chlorophyll with 
folds (/) of the cell-wall ; t pit-like formations (the 
contents of the cells contracted by glycerin, and con- 
taining drops of oil) ( X 800). 
