24 
MORPHOLOGY OF THE CELL. 
cells lose their protoplasm and become filled with air and water, the thin cell-wall dis- 
appears (as in Fig. 24, E), and the two pits form a single cavity, which is bounded 
by the over-arching thickening-masses, and is united with the adjoining cell-cavities 
by a circular opening (Fig. 24, A, D, E). In Pinus syl'vestris the pits are large and 
distant from one another, and the whole process may be easily traced step by step. 
The process appears somewhat different when pits he very near to one another, 
as in Pitted Vessels. In this case the thickening first appears in the form of a net-work, 
of which the thin parts of the cell-wall occupy the polygonal meshes, as may be very 
easily seen in young Maize-roots, for instance. Fig. 25, A, represents a portion of the 
side-wall of an already mature vesseP of the root-tuber of the Dahha. The ridges 
which first appear on the cell-wall are indicated at a, and are left white ; they enclose 
elliptical spaces pointed at both ends. As the thickening continues, the free edge of 
each ridge, as it grows further inwards, spreads, and becomes arched over the thin parts 
of the cell-wall. In this case, however, the overarchings do not grow uniformly, but 
in such a manner that their edges form at length a narrow fissure (r, in A and B). 
Fig. ■z\.—A a parenchymatous cell from the cotyledon of Phaseoiiis iiutllißorits FlG. 22.— Hypodermal cell of 
isolated by maceration ; i i the parts of the cell-wall where it is bounded by inter- the underground stem of Pteris 
cellular spaces ; / t cell-wall furnished with numerous simple pits, but not greatly aguilzna, isolated by boiling with 
thickened ; the thinnest parts of the pits are figured dark. B epidermis (e) and potassium chlorate and nitric acid ; 
coUenchyma (cl) of the leaf-stalk of a Begonia; the epidermal cells are thickened it is more strongly thickened on the 
uniformly on the outer wall, but where they adjoin the collenchyma only at the left side; the unthickened spaces 
angles where three cells meet; these thickenings have great capacity for swelling ; appear as branched canals (X 5So). 
chl chlorophyll-granules; / parenchymatous cell (X 550). 
Here also, when two similar cells adjoin, the same process takes place on both sides 
of the partition-wall, and lenticular spaces are formed by the overarchings ; these are 
at first bisected by the original thin cell-wall, which afterwards disappears, and the 
two cell-cavities are placed in communication at each bordered pit ; the canal or 
bordered pit which unites them is wide in the middle, and opens into each cell by 
a narrow fissure (Fig. 25, iJ, G). If, on the other hand, a vessel of this kind adjoins 
a parenchymatous cell which remains always full of sap and closed, the thickening 
and overarching of the pit occur only on the side of the vessel (Fig. 26, F), the thin 
parts of the cell-wall remaining intact ^, and the bordered pit remains closed ; from 
^ For the definition of a vessel see Chap, ii, 
^ These thin pieces of cell-wall which close up bordered pits may, by rapid surface -growth, 
form bag-like protrusions, which grow through the pores of the pits into the vessels, spread them- 
selves out there, become divided by septa, and thus form a thin-walled tissue, which not unfre- 
quently fills up the whole of the cavity. These formations are known under the name of 'Tüllen' 
or * Tyloses ; ' they are abundantly and easily seen, for instance, in old roots of Cucurbita, and in the 
