220 MiyUTE ANATOMY OF FLOWEIUyG PLANTS 



ott8. Cross section of 

 collenchyma. 



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

 distinctive glistening ap- |(a^___^ 

 pearance. (.'ollenchyma 



— tissue composed of 

 such collenchymatous cells 



— is one kind of strengtli- 

 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 i-elh, with ver}' 

 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). 

 tShells of nuts also 



;'ive good illus- 



M'/J. Longitudinal 

 section of 

 collencliyraa. 

 The lens- 

 sliaped l)od- 

 ies are clilo- 

 roptiyll gran- 

 nies. 



370. Grit cells from a pear. 



tralions of cells 

 with "walls simi- 

 larly thickened, 

 and affording pro- 

 tection by consequent firmness. 

 511. Cell union, or fusion, is 

 illustrated in the case of many ducts, in which 

 it is impo,ssibIe to distinguish the original cells, 

 placed end to end. Tlie ducts of the wood are 

 tubes giving unbroken communication between 

 the absorbent roots and the leaves. The walls 

 may remain relatively (liin ; 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, Jatex ttihes. holding the milky juice of Poppies, 

 Dandelions, and allied plants, are formed from originally 

 distinct cells by the Ijreaking down of intervening walls 



Fd 



