72 



MORPHOLOGY OF TISSUES. 



the interior of the substance of the cell-wall. Thus a fissure arises which, cor- 

 responding to the relationship pointed out, assumes the form of a triangular prism 

 with concave sides (Fig. 58, z). It becomes filled with air, and now 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 confine 

 the intercellular space grow rapidly, and thus it increases in width ; the cells assume 

 irregular outlines, or appear star-shaped in transverse section, touching one another 

 only at small portions of the surface (as in the parenchyma on the under side of 

 many leaves of Dicotyledons, and the stems oi /uncus cffusus). In the middle also 

 of the faces of the cell, when no other wall intersects them, splittings of the homoge- 

 neous lamella may occur locally ; sometimes these are limited to narrowly circum- 

 scribed places, which can then be recognised as shallow excavations in the otherwise 



Fig. 59. — Two rows of cells running in a radial direction 

 /, //, /// and I, 2, 3) of the cortical parenchyma of the 

 root of Sagittaria sagittafolia in transverse section ; a the 

 protrusions, e the cavities between them (X about 350). 



Fig. 60.— From a tranverse section of the leaf of 

 Pinus finaster ; h half of a resin-passage, to the left 

 parenchyma-cells containing chlorophyll with fold- 

 ings-in [/) of the cell-wall ; / pit-like formations (the 

 contents of the cells contracted by glycerine, and 

 containing drops of oil) (X800). 



homogeneous partition-wall. In other cases the splitting of the partition-wall into 

 two lamellae takes place in such a manner that only single roundish places remain 

 unsplit ; the separated pieces continue to grow rapidly by intercalary growth, and 

 bag-shaped protrusions of the neighbouring cells arise which allow the originally 

 unsplit fragments of cell-wall to be still recognised between them as partition-walls 

 (Fig. 59). In other cases there follows on the partial splitting of the partition-wall a 

 local growth of the two lamellae (or of only one) of such a character that a folding-in 

 arises, which intrudes into the cell-cavity, as shown in Fig. 60, f. Finally, in 

 some species of the genus Spirogyra, the septum between each pair of cells splits 

 into two lamellae, each of which grows in a peculiar manner ; a protrusion is formed 

 into the interior of the cell, which, when the adjoining cells separate, becomes 

 turned inside out somewhat like the finger of a glove previously folded in. When 

 the simple walls of cells united into a tissue split everywhere into two lamellae 

 (the separation proceeding always from the original intercellular spaces) and 

 become rounded off, a complete dissolution of the tissue takes place in this man- 

 ner, and the tissue becomes a mere mass of isolated cells. This occurs in the 



