220 MINUTE ANATOMY OF FLOWERING PLANTS 



368. Cross section of 

 collenchyma. 



369. 



369). These walls, when seen in cross section, have a 

 distinctive glistening ap- 

 pearance. Collenchyma 



— tissue composed of 

 such collenchymatous cells 



— is one kind of strength- 

 ening tissue. It is to be 

 found near the surface 

 of herbaceous stems, of 



petioles, and of leaves, along the midribs. 

 510. Grit cells, or sclerotic cells, with very 

 much thickened hard walls, are exemplified 

 in the rind and external flesh of the pear, 

 where they occur in groups. The walls 

 are traversed by canals, of the same nature 

 as the pits spoken of above (Fig. 370). 



Shells of nuts also 

 give good illus- 

 trations of cells 

 with walls simi- 

 larly thickened, 

 and affording pro- 

 tection by COD sequent firmness. 

 511. Cell union, or fusion, is 

 illustrated in the case of many ducts, in which 

 it is impossible to distinguish the original cells, 

 placed end to end. The ducts of the wood are 

 tubes giving unbroken communication between 

 the absorbent roots and the leaves. The walls 

 may remain relatively thin ; in this case they are 

 braced internally by rings or spiral thickenings 

 (Fig. 371). The ducts take their names from 

 their markings, being designated as annular, spi- 

 ral, or pitted ducts, etc. 



512. Milk tubes, or, in more technical lan- 

 guage, latex tubes, holding the milky juice of Poppies, 

 Dandelions, and allied plants, are formed from originally 

 distinct cells bv the breakins: down of intervening walls 



Longitudinal 

 section of 

 collenchyma. 

 The lens- 

 shaped bod- 

 ies are chlo- 

 rophyll gran- 

 ules. 



370. Grit cells from a pear. 



371. Spiral 

 duct. 



