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, ^, D, E). In Pinus sylvestrts 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 lie very near to one another, 

 as in Pitted Fesse/s. 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, ^, represents a portion of the 

 side-wall of an already mature vessel ^ of the root-tuber of the Dahlia. 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 (c, in A and JB). 



Fig. 21.—^ a parenchymatous cell from the cotyledon of Phaseo/iis ntultijlorus 

 isolated by maceration ; i i the parts of the cell-wall where it is bounded by inter- 

 cellular spaces ; 1 1 cell-wall furnished with numerous simple pits, but not greatly 

 thickened ; the thinnest parts of the pits are figured dark. B epidermis (e) and 

 coUenchyma (cl) of the leaf-stalk of a Begonia; the epidermal cells are thickened 

 unifonnly on the outer wall, but where they adjoin the collenchyma only at the 

 angles where three cells meet ; these thickenings have great capacity for swelling ; 

 chl chlorophyll-granules ; / parenchymatous cell (X 550). 



Fig. 22.— Hypodermal cell of 

 the underground stem of Pteris 

 aquilina, isolated by boiling with 

 potassium chlorate and nitric acid ; 

 it is more strongly thickened on the 

 left side ; the unthickened spaces 

 appear as branched canals (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, 5, 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, T), 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 ' Tiillen ' 

 or ' Tyloses ; ' they are abundantly and easily seen, for instance, in old roots of Cuairbita, and in the 



