^9 



ANATOMY AND PHYSIOLOGY OF 



Mg. 35. 



Intercellular passages occur mostly between parencliymatous 

 cells; they are frequently absent from prosenchyma, or when pre- 

 sent, are, at least, very narrow. They are closed in most places at 

 the surface of the plant, since the parenchymatous cells which form 

 the outermost layer of the plant are, in general, and in all parts 

 growing under ground or in water without exception, accurately 

 in contact at their angles ; on the other hand, on most organs ex- 

 posed to the air, especially on the lower sides of leaves, there 

 occur little ox^ifices bounded by crescent-shaped, curved cells, sto- 



mates or stomata (fig. 85), which allow 

 a free communication between the air 

 contained in the intercellular passages 

 and the atmosphere. 



The more regularly polyhedral the 

 cells are, the more do the intercellular 

 passages take the form of regular, nar- 

 row canals (see fig. 7) ; on the other 

 hand, the more globular the shape of 

 the cells (fig. 6), and in a still higher 

 degree, the more an unequal growth 

 has caused them to approach the form 

 of the stellate cell (fig. 10), the more 

 do the intercellular passages take the 

 form of irregular cavities, and the more 

 spongy becomes the tissue of the organ 

 formed of such cells, since the space 

 occupied by the intercellular passages 

 then becomes more equal to, or in ex- 

 treme cases, many times surpasses, that 

 filled by the cells. The lower side of leaves and corollas are 

 formed of such tissue, moderately spongy, the pith of Juncus 

 effusus gives a very highly developed example. 



In other cases the intercellular passages lying between regular 

 polyhedral cells become expanded at particular points into larger 

 cavities, or into long canals, which latter are frequently interrupted 

 at certain distances by partitions composed of stellate cells. This 

 is the case in the stem and in the leaf-stalk of many water- and 

 marsh-plants, in which the wide, regular air-canals are often sepa- 

 rated from each only by a single layer of parenchymatous cells ; 

 there also exists a roundish air-cavity (breathing -cavity, Ath- 

 Whungshohle) beneath each stomate. Canals and cavities of this 

 kind serve in other cases as reservoirs for peculiar fluids secreted 

 by the neighbouring cells, e. g., for balsams in the Coniferse, for 

 etherial oils in the IJmbelliferge, Aurantiacese, &c., for gum in the 

 Limes, Gycadese, and for milk-sap in Bhus, 



In many cases the spaces between the cells are filled up with a 

 solid matter, the intercellular substance, which is secreted by the 

 cells upon the outer surface, and sometimes only imperfectly fills 



Epidenixis of tho lower face of EelU' 

 hoTusfcetidus a, stomate. 



