66 GEOUPS OF TISSUES, OR TISSUE SYSTEMS 



Fig. 34.— Starch 

 storage cells of 

 potato. 



tions of the plant and of giving buoyancy to the part in 

 which they occur. 



97. Another important function of tissues is that of 

 storage of food substances. Storage tissues are usually 

 composed of large parenchyma cells 

 with large central vacuoles and compara- 

 tively little protoplasm. In some special 

 cases where the storage product is one 

 of the hemicelluloses this is deposited 

 against the cell wall forming a sort of 

 sclerenchyma tissue. 



98. In many plants a"re found secretory 

 cells. These often line closed cavities 

 (or "reservoirs") or elongated passages. These cavities 

 or passages may be formed simply by the pushing 

 apart of certain cells as the secretion is poured into 

 the space between them (i.e. produced schizogenously) 

 or certain cells may be dissolved, forming "lysigenous" 

 cavities. Good examples of 

 the first type are shown by 

 the gum canals of the ivy 

 {Hedera helix) and the tur- 

 pentine canals of conifers or 

 the glands of the leaves of St. 

 John's wort (Hypericum). 

 In the leaves and fruits of Rutaceae the cavities more 

 often arise by the dissolving of the secretory cells thus 

 setting free the secretion within a cavity. The secretions 

 are usually gums or ethereal oils, often containing resins, 

 etc. Other cells containing crystals of calcium oxalate 

 and other substances, perhaps including tannin, may 

 possibly be classed as excretory organs in which the 

 excretions are stored up in the absence of any structure 

 that would permit their being thrown out of the plant. 



FiQ. 35. — Gum and turpentine 

 canals of ivy and pine. 



