SUNFLOWER STEM 65 



walls at the points where three or more walls meet : 

 in the living state they are filled with air, and even in 

 specimens which have been treated with alcohol, air- 

 bubbles may still be found entangled in them. 



Observe carefully the resin-passages, which occur 

 in the cortical parenchyma. The resin, being soluble 

 in alcohol, has been removed. To see it in its 

 original condition sections may be cut from the fresh 

 stem, and stained with tincture of alkanet. They 

 are intercellular spaces, formed by the splitting of 

 cell-walls. The cavity thus formed is surrounded by 

 small, thin-walled epithelium, the cells of which 

 divide both radially and tangentially as regards the 

 passage. 



The development of the resin -passages may be observed with 

 great ease and certainty in transverse sections of the stem of Ivy 

 (tledera Helix). Cut transverse sections from a stem in which 

 the development of cork has gone so far that the epidermis is 

 beginning to peel off: mount in glycerine. Scattered through 

 the cortex and pith will be found passages already well developed, 

 and having a structure similar to those in Helianthus. If the 

 soft bast, which lies immediately outside the cambium, be 

 examined carefully, resin-passages will be found in various stages 

 of development, starting from a group of four cells, with no 

 intercellular space. In older stages the cell-wall will be found 

 to have split at the angle where the four cells meet, while in 

 older stages again the intercellular space appears larger ; irfean- 

 while divisions (radial and tangential, the former more frequent) 

 occur in the epithelial cells. 



Treat some sections with some staining reagent, such as carmine 

 or hsematoxylin, and note that the protoplasm of the cells is 

 stained, but no such staining is seen in the resin-passage : this 

 indicates the absence of protoplasm in the passage itself. 



Reactions of Resin, With a razor wet with water cut trans- 

 verse sections from a fresh young stem of Ivy : mount in water, 



F 



