TISSUE. 



89 



serve as resin canals (Fig. 202, transverse section) ; in the 



Composite, where they 

 are oil-passages ; and in 

 the Umbelliferce, -where 

 they are filled with a 

 mixture of gum-muci- 

 lage, and oily or resin- 

 ous substances. The 

 adjacent cells may as- 

 sume the function of 

 glands, and secrete the 

 202 substances found in the 



canals. The large air canals in many water plants are 

 intercellular spaces, whose origin is referable to the split- 

 ting apart of cells. But the large cavities in the stems of 

 many plants (as Grasses and Umbelliferee) arise by a break- 

 ing of some of the cells. 



114. Although the several kinds of tissue merge into one 

 another, they may generally be referred to two typical 

 forms, namely, parenchyma and prosenchyma. In 

 parenchyma the cells are polyhedrical and generally 

 isodiametric ; or, if elongated, the ends are truncate, or, at 

 least, never pointed and 

 wedged into one another 

 (Fig. 203). The tissue of 

 some Fungi, formed of hy- 

 phse, or threads placed close- 

 ly together, resembles paren- 

 chymous tissue, and is called 



' Fig. 202. Transverse section of a Pine leaf, showing a Resin canal (r.c); st, 

 stomata : hy, hypoderm ; cA/, chlorophyll ; a, air-cavity. Fig. 203. Section of a 

 Potato leaf, showing Parenchyma ; e^, epidermis ; l«, intercellular spaces ; cA/, 

 chlorophyll. 



