68 Plants and their Ways in South Africa 



with hydrochloric acid, showing that the cellulose is encrusted 

 with carbonate of lime. At times there is an abundance of 

 lime while again it disappears ; it may therefore be an ac- 

 cumulation which is secreted and may be of further use, though 

 it also is commonly regarded as an excretion or waste product. 



6. Resins and Ethereal oils, which give odour to many 

 plants, are probably excreted products. They remain in leaves 

 and are cut off at leaf fall. 



7. Tannin-containing cells are found in abundance in 

 epidermis and cortex ; tannin is probably a waste product. 



8. Enzymes which serve to digest food substances and 

 render them capable of absorption are secreted in special cells 

 in Cruciferae, Capparidaceae, and other orders. 



To summarize : — 

 Cells may be — 



1. As to chemical composition, of cellulose, lignin, suber, 

 cutin, mucilage. 



2. As to growth forms parenchymatous, about as long as 

 broad, not pointed (rounded, oval, tabular, flattened, stellate). 



Prosenchymatous, long and pointed. 



3. As to thickening — 



{a) Not forming patterns — 



Collenchyma, angles thickened, unlignified with 



contents. 

 Parenchyma — 



Sclerotic cells, uniformly thickened, lignified 



without contents. 

 Thick walled parenchyma, uniformly thickened, 

 retaining contents. 

 Prosenchyma — 



Sclerenchyma, uniformly thickened, lignified, 

 without contents. 

 {b) Forming patterns — 



Parenchyma with simple pits found in wood, 



phloem, cortex, pith. 

 Prosenchyma and vessels, forming rings, spirals, 

 simple and bordered pits ; found in wood ; in 

 tracheae and tracheids. 



