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CHAPTER VI. 



ISOLATED TISSUES OR CELLS HAVING A SPECIFIC 

 FUNCTION. 



The tissues and cells which will now be described are, so to 

 speak, only isolated in so far as they have special functions to 

 perform. It should, however, be clearly understood that their 

 protoplasmic contents communicate with those of the cells of 

 surrounding tissues, and that no living cell in a plant can be 

 looked upon as being completely isolated from the other cells of 

 the community. 



Under the above heading will be studied : — 



(a) Seeretopy cells of oil-glands. 

 (6) Resin-eanals. 



(c) Cells in which mineral matters may separate out under certain 

 <;onditions. 



{d) Idioblasts. 



(c) LatieifePOUS cells and vessels. 



A. Secretory Cells of Oil-Glands. 



Oil-glands are of wide occurrence in the higher plants, and 

 may be found in almost any position in the stems, leaves, or in 

 connection with the parts of the flower. The essential cells of 

 any gland are the secretory cells, which, as a rule, line a central 

 cavity as a layer one or two cells thick, into which cavity a 

 special oily secretion is poured, or freed by the breaking down of 

 the secretory cells. 



One type of such a gland occurs in the outer layers of the 

 cortex of fruits belonging to the genus Citrus (Citrus aurantii). 

 If a thin section be taken of the cortex in a direction perpen- 

 dicular to the surface, the following structure may be made out, 

 using a low power of the microscope : — 



i. Externally, the epidepmis of the fruit. 

 ii. Internally, cells of the eOPtex (pericarp). 

 iii. The oil-g^lands lying quite near the surface. 



