THE STE&[ 



101 



tion, their arrangement suggests a grotesque resemblance to 

 the face of an old woman wearing a pair of enormous specta- 

 cles and surrounded by a cap frill of netting with very wide 

 meshes. These are parenchyma cells, /, /, Fig. 115, and 

 constitute the greater portion of the living tissues. 



The two large openings, m, m f , thaJwpresent the spectacles, 

 are ducts for carrying water up tne stem. They are called 

 pitted ducts on account of the bordered pits which cover 

 their outer surface. The two smaller openings between and 

 slightly below the pitted ducts are also vessels for carrying 

 liquids up the stem. The lower one, a, is called the annular 

 tracheid because its tube is strengthened by rings on the 

 inside. The upper, smaller one, sp, is known as the spiral 

 tracheid, because its walls are reinforced by spiral thickenings. 

 Can you think what is the use of these strengthening contri- 

 vances in the walls of conducting cells? (Suggestion: What 

 is the use of the spiral wire on a garden hose?) The large, 

 irregular opening below the ducts is an air space. What is 

 its object? Why has it no surrounding wall? 



Next look above the ducts for a group of rhomboidal or 

 hexagonal cells, v, v, with smaller ones, s, between them. The 

 larger of these are sieve tubes, the smaller 

 ones, companion cells. The sieve tubes 

 carry sap down the stem after it has been 

 made into food by the leaves. They get 

 their name from the sievelike openings 

 between the connecting walls of the cells 

 which form them as if a row of pepper 

 boxes with perforations at both top and 



i -i i -i mi view ui nit; aievu tuuu 



bottom were placed end to end, so as to of a gourd stem, showing 

 form a long tube divided into compart- P erforations - 

 ments by perforated walls. Can you give a reason why the 

 cells of ducts that carry elaborated nutriment should have a 

 more open line of communication than those carrying crude 

 sap ? [56 (2) .] Which one of the organic food substances was 

 shown by Exp. 39 to be unable, or nearlv so, to pass through 



FIG. 117. Horizon- 

 tal view of the sieve tube 



