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 o[ /uncus effusus. In the faces of the cell, where no other wall intersects 

 them, splittings 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 



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 sagittifolia in transverse section ; 

 a the protrusions, / the intercellular spaces between them 

 (X about 350). 



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

 Pinus 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) (X8oo). 



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 lamella (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 



